Lately, I haven't been able to do more than a couple hours a day, if that. I'm starting to twitch right now, actually, not having played seriously since Saturday....
If you want real addiction, look at a game like Everquest or some other MMORG. Six-eight hours at a time is nothing. Powergamers (and college students, I guess) will play 18-20 hours a day, in one sitting. Then get up and come back for more.
I play Civ 3 because my Real Life couldn't support the kind of addiction necessary to compete.