http://patchtime.wowachiever.com/ shows each of the patches, when they went into PTR and which dates they went live. Only one or two ever exceeded 8 weeks. This seems to be a very consistent 45-60 day testing timeframe for Blizz.
Wasn't there a twitter post from Ghostcrawler or someone else saying that when everything goes the way they planed they'll need about 2 months for a minor content patch and about 4 month for a major one. MoP was released end of September and 5.1 was released in December which are about 2 months. Taking the 4 months from the twitter post the patch should be released in end of March or beginning of April. There's much more to test than in the past. We have a new daily hub, two new world bosses with a new factionbased loot mechanic, new rares designed to be taken down by 5man groups, new scenarios and a new raid. That's a lot of content. Patch 4.2 only had a raid and a daily hub and 4.3 only had one raid and 3 new 5 mans. At the moment there's nothing new to test on the PTR apart from the world boss, which probably won't drop loot yet (dungeon compendium doesn't list any loot).
I think the earliest it could possibly be released is Valentine's Day week, 6 weeks from today. In case it's close, I'd advise serious raiders who have actual human significant others to make plans during the first or second weekend in February, in order to get enough girlfriend/boyfriend points to ignore the relationship for a week or two once the patch drops.