It depends on what you mean, If you mean logged on in general probably 10-12 hours ( I don't turn off my browser ) if you mean actually looking on it. Maybe 15 minutes
If I'm selling something then probably half an hour to an hour a day. If not then I visit two or three times a week just to have a look around or make the occasional post. I have a 75 hour week so find little more time than that to use the site (unfortunately)