How long does it take to get to level 90 from 85?

[Sarumonin]

D: I cant wait for the new level cap

[Lochton]

What? 85 to 90, about 6-10 hours on a 'casual' base of play time. Depending on if you are one of the classes that can't roam about Timeless Isle.

[Forgettable]

If you're efficient, it should take approximately 2 hours per level. So 10 hours give or take depending on if you dick around or not.

[Belerophon]

I just finished my last level 90 to get one of every class and this is the route I took:

-Get to Pandaria and run to Dawn's Blossom to buy gear.
-Go back and level through Jade Forest until you hit 86 (stay queued in LFD and rested for a good amount of xp boost).
-At 86 go to Kun Lai and level to 87 (don't forget LFD when available)
-At 87 go to Townlong Steppes and buy any gear you may be behind on and do the zone in it's entirety as it flows fairly well.
(If you queue for LFD at this point and do not have the updated gear you may run into trouble as a healer or tank)
-This zone should take you about halfway through 88 if not more so should you remain rested.
-Once Townlong Steppes are done head into dread wastes and quest until you hit 90. (usually hits around Soggy's Gamble)

Selling any outdated gear or drops will give you enough gold to afford flying as well so you should be set.
Some people swear by farming the caterpillars by gu'chi on timeless isle but I did not find it successful unless I had to shave off a few bars at 89.

[Ciddy]

Not really that long. I didn't keep track of time played per level, but I suddenly got an urge to level my 85 alt a week ago, and he's 90 now (playing on and off, about every other day). A friend of mine also had an alt that level, so we did all our questing together.

[Attackrabbit]

As long as a piece of string