Between 30 mins and 1 hour, i'm reading new threads, posting, giving thanks, etc... I think it's ok. Sometimes i'm visiting the site more than once a day.
I come in several times a day to look over the new posts.
All in all, probably more than an hour.
Once a week, but I spend at least three hours when I do come on. This place is so damn addicting- I just don't have the time.