This could be useful for a "1000 ways to know you've played CIV3 too much" thread.
I used to play CIV1 for up to 14 hours, now it is impossible for me to keep the pace due to several reasons (work, sleep, wife, etc.), but I stop playing when I start to get confused about if I am playing CIV3 or street fighter, I mean, when I start punching everything and throwing nukes at everyone. Then I know the little guy inside my head realized that my patience and concentration are over. This usually happens after 6-7 hours. I also stop when external factors (wife) makes impossible to keep playing.
Also when I go to sleep I see the settlers moving... and at work when I want to take a break... yes, I think of the moves I will be making when I get home.