I'm not sure what link the dogs posted, but a rule laid out by Howard Lederer is that he won't lose more than 30 big bets in a session. If you're playing .5/1, and you go 30.00 down, stop. There are many reasons, but the biggest I can recall is that it keeps your table image up. Remember, you're playing in the same limits usually so you will probably see some of the same people that you saw on your losing night. Don't ruin your image throwing good money after bad. Pack it up and move on. You keep your shirt, and your table image.
I just read the article the dogs posted and it was the same one I was thinking. Damn, four dogs, we run in some of the same circles?