When Do Pros Quit a Cash Game?

Paul Phua poker asks the question we have all been waiting to hear "When do you quit a cash game?". We have all been in the situation where the game is good but we're tired or the game is bad and we're losing...so when do the best in the world know when to stop? 

Asking pros like Holz, Trickett, Phua, TrueTeller and O'Dwyer the common answer seems to not to be about how much they are winning or losing but more based on how they are feeling!