I can never quit a session ahead. Yesterday for exAMPLE, I was up 8 buyins in 2.5k hands. I continued to play, took some bad beats, tilted, moved up, got coolered blew half my roll.
So for now on when I am up 6+ buyins I am quiting for 1-3 days(Most likely just 1). When I am down 6 buyins I am quitting for 3-7 days.