I play a TAG game at 5 or 6 handed tables. I think you are correct in saying stop before I get frustrated, but I just wanted to see what others did.

I use Mark Caro's Pain Threshold rule to tell me when to get off tables, during both good sessions and bad sessions. When I stop feeling the pain of additional losses, or the enjoyment of additional gains, it's time to get off and take a break for a while.

If things are going badly, I quit, and don't get back on until I'm 100% sure I have my emotions under control and I'm not craving action. When jumping back online the same day as a big losing session, I also take the time to make 100% sure that the tables I'm joining are as juicy as possible. If there aren't enough tables that look good, I'll usually just take the rest of the day off.

My good days are almost always much better than my bad days are bad ... so this works for me. Your mileage may vary and all that.