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?