I get that message sometimes. I think literal action is necessary to be kept logged in. Just reading or even typing does not do it, one has to literally hit a post button or switch threads one is reading, etc. If one is reading a very long thread with lots of replies and then typing a longish response, there is no way for the site to know you're there. Scrolling to read and typing in this message box does not get to the server.

I rarely log out of the site, just "leave" and so the site has to log me out with the timer. I wouldn't want it set any longer as I wouldn't want to be listed as here when I'm not. I think too, how many people can be on the site at once is limited and so the timer needs to be set as it is so at peak times, people not responding are logged off and new ones allowed to enter. But I think too, that's partially why the length of threads is limited to 100 posts.