How do you learn when to walk away from a cash game?

So ive had that problem in the past, and i think i found a decent solution to it.

What i do is i bring a certain amount of $ to spend, and dont spend more then that (obviously).

Above that, whenever my stack gets above a certain designated amount of chips (usually my starting chip stack) i put those to the side, and do not spend them after that point. So i can keep a full stack of chips, and still i have an idea of how much im gonna walk out with if i lose them from that point on.

Anyone else have any kind of system like that?

I would suggest not physically putting them to the side, maybe know where you are at but putting them to the side gives off the impression that you dont want to use those chips and thus a bet to make you use those will get you off a better hand. I read this somewhere I think Gordon said it in Little Green book?

Depending home much I sit down with, I will leave a table as soon as I have doubled or tripled my stack. It will also depend on who is still at the table when I reach this point. If I am lucky to be sitting with players that are making it easy for me I'll stay a bit longer.

Also depends on limit or NL. Limit, you can put $$ to the side and know that when your limit bets dip into that stack you will leave on the next hand. NL you may be pushing the money on the side with an AI bet, so no need to put it to the side. I do the same when I play slots. Say I start with $5, "I'll stop when I hit $10", then I hit $12.50, "I'll stop when I hit $15, or go down to $10"... which is essentially the same as LHE. There's really no way to do this with NL since you may be pushing on a monster.

If you read Karo's Book of Tells, the set aside chips (winnings) is mentioned. Basically, if you want someone to call they will play looser with those winnings than with their normal stack.

I was flipping through Caro's book today.
Tell #1 has a pic of a guy with his chips and says, "Notice that there are a few extra chips on top of his large stacks? This could be his profit. That's important to know because you can frequently bluff successfully just by betting slightly more than his profits."

My basic rule of thumb is that I bring enough for 2 buyins. If i lose that, I'm done and I go home. If I end up tripling up what I brought (lol doesn't happen too often lets face it), then I go home.