It is difficult to tell. I'm logged in always when home or at office, both at Seven Forums and Eight Forums
. As I am insomniac and usually only sleep about 5 hours, it means I'm logged in at least on one of my computers something between 17 and 20 hours a day.
But the forum activity is then a totally different thing. For instance when working I mostly just check the RSS feed every now and then, picking most interesting headlines for deeper inspection. Same at home, the normal life, family, friends and neighbors have little bit higher priority than anything online, so any offline activity goes first.
This, constantly online but prioritizing offline activities over online stuff makes that sometimes I post / response only 10 to 15 times a week, sometimes a few times that in a single day. This is also the reason why I have decided against joining any other forums; time is limited and life offline important, better to focus on one (for me the SF & EF are one thing) instead of only using a few minutes on several forums.