How do you know when you've had enough of Azeroth and it's time to go do something else? Do you play for a certain amount of time every day? Do you PvP until your eyeballs get all red and puffy? Do you grind and grind until you at last have something to show for your effort?
Personally, on most days, I have lots to do, and commitments force me to say "I gotta go!" But on days when I have the whole day off and nothing specific planned, sometimes I feel like I can just let loose and play as long as I want. Other times I feel the need to limit myself to a certain amount of time with WoWTimer.
I try to listen to that little intuitive meter inside my body that is always measuring whether or not whatever I'm doing is still a good use of my time. Seriously, when I'm spending too much time on WoW (or on anything really), I get this feeling in my chest and in my back, something like a tensing of muscles that tells me I should be more productive. It's different from sitting in a chair too long... it's more psychological, perhaps.
How do you prevent yourself from getting carried away?