On the average weekday, I would say anywhere from 2-4 hours if I don't have anything going on. When I work from home, I can usually play for an hour in the mornings before work, and then it's pretty easy to do another 2+ hours at night, but if there's something good on TV (football, WWE, Survivor) I might not power through all night. On weekends, I would guess closer to 6-8 hours a day. I basically play 1-3 hours at a time, and do that in the morning, afternoon, and evening.
Amazingly, I do leave the house sometimes, so obviously these numbers might go down to 0 some days, but I like being at home so I'd say more days look like this than not.
Also, I find it depends on what game I'm playing. When I'm really enthralled in a game, it's easy to play for 3 hours straight, but certain games are tough to play for more than an hour at a time. Usually, I'll just stop and do something else rather than trying to juggle multiple games.