As far as I can tell, if I leave PC without logging out, or if I keep a PC page open but step away from the computer, or if I work busily on a post without clicking any links for other Forums pages, after an hour I'll automatically get logged out. The difference is that next time I do click on a Forums page, if I have the "remember me" box checked I'll get logged back in automatically and not notice I was ever logged out. If I don't have it checked, I'll get taken to a log-in page (and in the process may lose the post or PM I was working on).