Basics of the Ranking System
(This is heavily edited, the OP is at the bottom.)
Update Dec 1 2018: If you didn't play classic, this is the short version:
Hitting the top levels of the honor system frequently required 8-12 weeks of playing 14 hours/day for months at a time without missing a day. This is for a few reasons:
--Each week all players lost 20% of their accumulated progress.
--On an average server, only the top five players for a week would make more progress than they lost.
--Going from Rank 12 to Rank 14 took about six weeks of being one of those top five players.
--Skill barely mattered, it was mostly hours logged.
--Because of the absurd time investment required, people used multiple ways to make it more manageable, such as forming premades, forming cartels that limited play, and sharing their account.
More thorough explanation:
The math behind the system was punishing because at the top levels only the top .2% of players in a week could make significant progress on a process on which the final step took a minimum of 3 weeks. The minimum possible amount of time to go from character creation to High Warlord / Grand Marshal was 3 months.
Thanks to Ettan in post 37 for a good explanation, and http://wowwiki.wikia.com/wiki/Honor_...-2.0_formulas) for a very detailed explanation. (On that page, though, most the links are broken.) This is my explanation, which is more intended for players that did not play vanilla:
There were two sub-systems, that were very similar, used for determining ranking.
--One was "Ranking Points", which determined a character's title from 1 (Private or Scout) to 14 (Grand Marshal / High Warlord). It was basically reputation except it could go down. It took 60,000 Ranking Points to achieve title 14. A player could see their own Ranking Points by running a short macro.
--The other was a ranking system that also divided players in to 14 groups, but it was based on a player's contribution points for that week only. During server maintenance, a list of every player that had gotten 15* honorable kills that week was generated. Then it was sorted by contribution points earned that week. The players at the top .2%* of that weekly list earned 13,000 Ranking Points. Players in the next .5% got 12,000. Players in the third bracket got 11,000, and so on. The size of the brackets was very bottom-heavy: the five top ranks comprised only 7.7% of players, but the bottom five had 51% of players. This is the part of the system that pitted players against each other. *(the exact number was different in different patches)
Larger servers could have 4-6 people in this top bracket. Servers with few players could have zero players in this bracket. The breakpoints on a graph looked like this: http://wowwiki.wikia.com/wiki/Honor_...vsWS-examp.png
This weekly list is also why what it took for an individual to rank up could vary immensely from week to week and server to server. If a half-dozen people decided to get 1.5 million contribution points that week, a player would need to get 1.6 million points to beat them. If the players were organized and limited their play, it could be possible to top that list with far less.
--Ranking Points decayed 20% each week. This meant that a player 1,000 points away from title 14 (59,000 RP) would lose 20% of that, or 11,800 points.
Because the amount of weekly decay (~11,800/week) was so close to the maximum weekly earnings (13,000), it meant that the mathematical lower limit it took to get the 5,000 Ranking Points to go from title 13 to title 14 was four weeks of being in the top weekly bracket. As a result of this, players who were hoping to get High Warlord / Grand Marshal a month in the future could interfere with players trying to finish it the current week, so competition could be fierce.
Being on low population servers was even worse, because if the number of players on a server was below a certain threshold, zero people would be in the weekly bracket that earned 13,000, which meant that the decay of ~11,800 a week was almost exactly the same amount awarded to the top players (12,000).
So in summary the combination of three things is what made the process so difficult:
--The max progress per week was capped
--That amount was very close to the amount of decay
--Only a tiny fraction of players could got the max progress per week
Edit 4-22-17: If the math is kept the same, it actually might be more difficult to get HW/GM than it was in vanilla. The unsung hero behind the rank 14 math is large numbers of casual players getting 15 honorable kills/week. Depending on patch, every ~333 or 500 players that get 15 HK/week open up one spot in the top bracket. My logic is this:
--The servers will initially be very popular
--People that play the game excessively will make competition for the top brackets fierce.
--In 2004-2007, vanilla Warcraft was one of the best games available.
--In 2019, vanilla Warcraft will have much more competition.
--After a brief surge, there will be far fewer casual players getting 15 HK/week than there were in 2004-2007.
--Which will leave the system with a disproportionate number of extremely hardcore players but far fewer casuals, basically the worst-case scenario
However, I guess the previous scenario isn't what has happened...in similar situations...so perhaps more casuals will stick around than I thought.
Other notes:
The amount of contribution points earned from killing a player per day had diminishing returns, so killing a player more than 10 times in one day gave no additional honor. When battlegrounds went cross-realm in patch 1.12 it significantly affected how players would farm for the top ranks.
There were dishonorable kills and the way they were calculated in was brutal IIRC so players would not do anything that would risk getting DKs.
One other thing is that most game systems displayed a player's highest rank ever achieved, so if a player got Grand Marshal / High Warlord they got to display it forever.
In 1.12 the number of players at the top bracket was increased from 0.2% to 0.3%.
The rewards for getting rank were gear purchased from a PvP vendor for gold. The armor acquired at rank 13 was very good for most classes (I guess a little less so for casters), and rank 14 only additionally unlocked the PvP weapons. Apparently the weapons were cheat codes for fury warriors and rogues but much less impressive for other classes.
Post #69 from Katsu2881 also touches on some of the psychological aspects of getting so close to the reward, which caused people to pursue higher ranks via unhealthy play.
Random factoid: If I did the math right, the soonest possible week a HW / GM could appear on a new server would be after the reset of week 12 (i.e. week 13) if a person got 13,000 RP from the top bracket every single ******** week.
If week after week a player got more Contribution Points than 25% of players they would earn 1,000 Ranking Points a week. Their decay would equal their progress at 5,000 RP, which means they'd top out at the rather underwhelming rank 3. The player would also need to play in the next highest bracket for the very last "push" week or they would wind up with something like 4900 RP. This is generally not a big deal except for 13-14 where it's more serious.
...50% of players, 3k RP/week, 15k max RP = rank 5
...75% of players, 6k RP/week, 30k max RP = rank 8
...90% of players, 8k RP/week, 40k max RP = rank 9
...99% of players, 11k RP/week, 55k max RP = rank 13
...99.5% of players, 12k RP/week, 60k max RP = rank 13 but really close to 14
...99.8% of players, 13k RP/week, 65k max RP = rank 14
Again anyone that got 15 kills in a week was on the list, so it wasn’t super hard to play more than 50% of players. But when you start talking about consistently getting in the top 10% it was a pretty serious time commitment.
And to answer the title question, the answer in terms of hours played varies enormously. But it's worth noting that many people have replied it took 16/hour days for weeks.
Post #88 is very eloquently written and definitely worth a read. (page 5)
Also, if anyone wants to fill out / fix this template I'd be interested:
Honor Per Week / Rough Estimate of What It Took To get:
15 Honorable Kills: under 30 minutes
10k honor/week: A handful of battlegrounds each week.
100k honor/week: ~10 hours in a week casually
500k honor/week: ~8 hours/day day without a group or ~6 hours/day in solid pre-mades
1000k honor/week: ~16 hours/day 7 days a week in very good pre-mades
1500k honor/week: ~20 hours/day immediately winning battlegrounds
2000k+ honor/week: Continuously playing
Also, various posters have said that acquiring the top weekly reward was in the best case scenario 500k honor / week (with everyone at the top levels cooperating) and the worst case scenarios were in the 2,000,000 - 2,500,000 honor a week scenario (intensely competitive PvP servers with multiple farming groups competing instead of coordinating.)
Edit April 27:
One of the things I realized after writing this was that people that acquired Rank 14 didn't necessarily have to be good at PvP. I find this pretty ironic, as it was so difficult to get and required so much dedication, but it was really more about time commitment and farming more than being better than anyone else.
Obviously with the kind of time commitment that was required, R14s played far, far more than the average player and presumably got at least proficient. And if the player was really bad they'd have had a hard time getting in to a premade that could consistently win.
But again it's kind of amusing to think that even someone that was extremely skilled at PvP that "only" played 30 hours / week would probably top out around Rank 11 or 12.
OP:
Getting those titles was arguably the most difficult thing to do in Warcraft. The only things that come to mind are world-first kills (particularly things like pre-nerf M'uru) and some solo kills (Yogg) but...it's just kind of apples to oranges, as the PvP titles were the only thing that ever pitted people with crazy amounts of time to spend on the game against each other.
Which patch the game launches on will have a big impact on this, as http://wowwiki.wikia.com/wiki/Honor_system_(pre-2.0) lists that there was a change in Patch 1.8.
I was not a big PvPer myself but did read a post in vanilla that suggested that you needed to make a social network of all the people pushing for it. Then for the three weeks before the push come home from work and play until bedtime every day and pretty much continuously on weekends. Then the week of the push the person needed to take vacation and just play the entire week. All of this preferably being done in groups that were capable of winning battlegrounds, as it was much easier to rack up kills in good groups.
However, getting these titles is arguably the one thing that will be harder now than it was at launch, as players have optimized every aspect of the game to the smallest details over the last fifteen years...but that will be all put to the test as extremely experienced players square off against each other.
Also, if people want to post what they think it will take to get other ranks, especially the ranks that give gear for PvE progression, feel free to post that as well!