Set time guidleines and play to them and stop no matter up or down. Take all the gray area out of what you do in a casino. How you play, and how long you play should all be known before you hit the floor. Discipline is all thats needed to play your best game always. Compromising your play based on chasing losses, or trying to add on wins, when you're tired is not smart. As a counter your advantage is tiny, its not hard to lose it and not even know it.
These rules will vary some if you are doing things other than just counting. Some opportunities are fleeting and you have to grab them and play them when you can. As was said before, the casinos aren't going anywhere. Play smart. There are no heroes in blackjack. No need to play through fatigue, pain, or discomfort. It proves nothing other than lack of discipline. Marathon sessions are not for straight counters, if you think it is you probably are not playing with an advantage. Even if fatigue doesn't get you the casino will. And if the tap doesn't come after sitting there playing for 12 hours, you just aren't that good and the casino wants you there.