Yeah cash games are an "ongoing" ordeal, some ppl say play a set time, number of hands, winnings... it's up to you, I (personally) think that you have to set your standard. Up 100% buy-in, 200 Hands, whatever. I sort of play "if I'm up +100% buy in then I leave after a set number of hands" in my head. Usually I stay too long and lose most of it back. Otherwise I'm down X% and say to myself if I can just make a little back then I'll leave, and I do then I stay saying to myself "OK one more big win and I'll leave".
I really can't tell you, sorry. Knowing you are better than everyone else at the table may make you want to stay even though you are currently down. Maybe a winning player always quits once he (or she) is up 10% of buyin, everyone can get there unless they are really bad or on a really bad run. That would be a slow way to profit, say $5 buy-in and $0.50 profit each session, but you would be a "winning" player. It's up to you to decide, set a goal and STICK TO IT! Whether it's hands, time or money.