The only artificial limit I set on my sessions is that I generally try to call it a night if I lose more than 30BB or so. This is just to prevent myself from unknowingly tilting rather than any sort of money management scheme.

If I can honestly say I still feel good and the game is still good, I'll stick with it anyway. (I was down 50BB at one point in a 3/6 mixed game the other night, but the game was way too good to quit.)

Other than that, I play until I get tired, bored, or have to do something else.