Unfortunately, it is a depends thing with how long to let beer sit on yeast. It really depends on yeast health and probably the shape of the fermenter. With a flat bottom fermenter, you'll have more direct exposure to the beer with the trub at the bottom. I've had beers sit on yeast for a few weeks post crash without developing off flavors, but I wouldn't want to make that a regular practice.
For dry hopped beers, you've likely got everything you want out of the hops after five days to a week and will start to get things you don't want from the plant matter if you let it sit much longer.