If you're going to set limits to your sessions, it should be in terms of time instead of win/loss, and the motivation should be to only play as long as you can play well, not to play as long as you're winning or something.

Having said this, some players do have a stop-loss amount for their sessions, that is if they lose so much in a session no matter how they're playing they quit for the day, or for a few hours, or whatever. The reasoning behind this is that even if you're playing well, if you're not well-disciplined, losing a relatively large amount of money has psychological implications on your play.