I've often wondered the exact same thing. When I get sucked in and read and make a few posts, I find that hours have slipped by. I actually don't hit the forums too often because of my fear of commitment. I'm baffled by some of these prolific posters.
Several weeks ago I intended to start a poll on this where people could place themselves on a scale between "Rapid Posting Skillz" and "No Life". I never got around to it because I didn't have the hours to spare.