there are many reasons you should stop playing at a cash game
1. You are not playing at your best due to fatigue, other things on your mind
2. You no longer have an edge at the table
3. You're the sucker at the table
4. You've reached your stop loss
5. No action at the table
5. You want to stop playing
Number 5 seems to be really common sense but it is surprising how many people actually follow it. I have lost count of how many times I have said "this is my last orbit", "I would cash out, but I don't want to break the game", or "I'm going to stop at X time" only to find myself still playing long after my planned exit. When you want to stop playing, don't let anything stop you.