Yo Flex, here's my $.02-
I think that it's possible to tilt when you're winning in a cash game, because you feel invincible- especially at 300BB.... I think that this can be attributed to a mis-perception about how much money you have behind you when you are only at 100BB or less on a table. If you have $500 in your bankroll, and you buy in at a table for $25, how many chips do you have? You have $500 in chips until you lose or gain more chips. Do you see why? The table stakes (in this case $25NL) have merely capped the portion of your bankroll that you can initially put on the table. After all, you can constantly top off your chip stack to 100BB, so in reality you are playing with your entire bankroll behind you. Whether I am winning or losing at a table, I try to remember that my table stake is part of my entire bankroll. Here's a hypothetical- Say you took your $500 in chips and bought in at a $3/6 NL cash table, putting your entire bankroll at stake on the table. If you were to win a big hand and double up, the knowledge that the number of chips you held represented your entire bankroll would certainly discourage you from making rash deep stack decisions!
If I ever find myself in a frame of mind where I feel the urge to gamble, I ask myself, "Would I make this play while holding my entire bankroll on this table?" If I would not, then I am merely making excuses for a bad play since I will not "feel" the immediate effect (as I would if I had my entire bankroll at stake). If I cannot kill the urge to gamble, I immediately sit out and take a break!
How long I stay at a table-
Right now, I typically play 2 or 3 tables simultaneously, and I will sit at the same tables for the entire session. I think the longest I have been at one table is 6 hours! The only reason I will change tables is if someone on my buddy list has signed on, and then I will move to that table. If not, I just stay put, and adjust my play to the action at the table. I know that an argument against this is that tables can go bad, etc. The standard advice is to find the loosest table possible with mid to low stacks and go there. If the table tightens up, find another loose table. The fact that most players utilize this strategy has caused me to move in a different direction.
I have found that a typical micro or low-stakes online poker table will swing from loose to tight like a pendulum as time goes by. For example, I will sign on, find a loose table and sit down. Soon, the fish will bust out, and guess who will come in to replace them? Usually, these characters will be regulars, set miners, or other camping type players who are playing as many tables as they can, and got on the waiting list because the percentage looked good in the lobby. When all of the loose players have busted out or exited the table, the table becomes extremely tight (10% VPIP or less). This phase typically lasts only 15 minutes, and then the regulars get sick of passing the blinds around and leave. The table becomes shorthanded, and guess who sits down? Loose and sometimes desperate players who will sit down with weird amounts of money and lose it all. Then the cycle repeats as the percentage of players to the flop begins to look better and better in the lobby. In my experience, this strategy gives me more opportunity to play with shortstacks and loose players since it takes the lobby a while to register that the percentage of players to the flop is rising, combined with the fact that most multi-tabling regulars are not fans of playing at shorthanded tables, which I do not mind doing.
Long story short, the only reason I will cut a session short is if I am losing, and I feel that the downswing is affecting my play. At that point, I will switch to analyzing hands from the session to see where I went wrong, and then take a break and get back into the action. I hope this helps, man- Good luck!