Somewhere between 3 days and 3 months. If you're using a single fermentation vessel, you leave your beer there until it's done, which is subjective based on the results of your sampling.
The main reason for me is that there is no question that the beer is done at that point.
Anecdote: I have a tripel that's right at 7 days, and finished fermenting (probably done, at 1.011...from ~1.080). I'm tasting the hydrometer sample right now and I can't detect any hot alcohols. Obviously it's still yeasty, so it's not perfectly smooth. I'll leave it for another week on the yeast, but that'll mostly be at cold temps to crash out the yeast. I'll, perhaps obviously, store it cold in the keg for a while (month or two probably). I used 2 smackpacks (Rochefort yeast) in a ~1.3 liter stir-plate starter...about 80 seconds pure O2 before pitching). I started at about 64 degrees and ramped up to about 72-73. I'm psyched about this beer if you folks couldn't tell.
somewhere between 3 days and 3 months.
Wouldn't taste and a couple of gravity readings tell you if it's done before then?