It varies depending on what my other priorities are for the day. If i am working that day, I will probably pop on for half an hour at the most, just to check comments, and answer a couple of questions. I will also see how my traffic is going.
On a day like today, which is a holiday day, I have spent perhaps 2 hours on here in total? I am in the middle of writing a hub, have answered a few questions, replied to some comments, and participated in the forum.
It can sneak time away from you, i must admit, but I quite like it :)