[Kovner]

Why is it important to win chips early? You could have just bought those chips.

Anyways, people say you remember your losses more than you remember you wins. Its true. Another thing is that those sessions where you go up then end up losing stick out in your mind, because at first you are so happy you are winning, then when you lose, that happiness goes away PLUS you have to deal with losing. You think to yourself "if I would have just left at x o'clock, I woulda won."
Another reason this form of selective memory is prevalent is that it allows you to ignore your results. You look at your "poker diary" and it says you're a loser in a game, but you just say "well - I was ahead on these sessions"
Thoughts like these are falacious and will lead you on the wrong track of thinking about poker.
You need to be able to know when/where you play your best poker and you need to be able to separate yourself from how much you are up or down in the middle of a session. After that, your main concern will be "am I a winner in this game."
Learning how to answer that question will go a lot further than trying to set artificial stop-wins, stop-losses, or stop-hours.