One way to get away from this is to use the 'Remember me' option when you log in. This prevents your session from timing out.
Otherwise, you will be logged out automatically after 15 minutes from the last click you issued. How long is this preset time? I *think* it's 30 minutes. I'm gonna see if I can find the number for sure.
Basically though, consider this. If you click on a thread that has 8 posts you've not read.. and you completely catch up, then use the 'Quick Reply' box at the bottom of the thread... However long it took you to catch up, AND how long it takes to compose a response... add up. If that combined time is longer than 15 minutes, you're logged out.
Lemme know if this helps!
LK