The session timeout is exactly 300 seconds.
Anyway, if you hate IE and you use FF you may try: THIS
FF RealoadEvery addon. It's only 30 KB.
You just right click on a page, and set the custom reaload period, or use some of the pre-defined. As simple as that. Setting it on the forum would be forcing all users to reload the page, this way it will be up to users choice, if using FF of course. I don't know for other browsers becase I dislike them. Btw, if you are replying to a thread, and in meanwhile someone submitted a post while you were writing yours, the forum will notify you.