You should only stop playing for psychological reasons. Whether it's because the money you have won is important to you, you don't feel like losing more money, you just don't feel like playing, or you are tired and unable to concentrate.

You only have to stop when something (usually psychological) is affecting your play, if you are playing right then losing or winning money isn't important on any given session.

This will sound a bit redundant, but assuming you are a winning player, you make money by playing more hands. The only reason to stop playing is if you can't be a winning player for some reason during that session (ie. tilt).

I always keep playing unless i don't feel like it, because i know that i'm playing winning poker, and losing my winnings or increasing my losses is unimportant to me.

Its hard to explain, but put simply, it only matters that you are playing solid poker all the time, it doesn't matter what happens on a given session.