My big beers will be in fermenters for about 2 months. This gives plenty of time for the beer to clear and condition some but ensures there is still plenty of viable yeast available in the beer for bottle conditioning. It also frees up the fermenter and space in a reasonable time.
You probably could leave it in the secondary for as long as 6months without much problem but I don't have quite that patience. I want to start sampling a couple bottles before then.

If you leave it 6months or longer you should consider adding a small amount of yeast at bottling to ensure carbonation.
Craig