I'm online for a lot of the day anyway when I'm working. Popping in here every now and again is equivalent to chatting over the desk between tasks. I'm often up late, too, as I don't sleep well, and logging on is better than watching late night TV, which is pretty awful.
I've never thought to time myself, as I feel no compulsion to be on here. I would worry, though, if I found myself noticing how much time other people were spending on here - now that's obsession .