from vbulletin: Admin CP -> vBulletin Options -> Cookies and HTTP Header Options -> Session Timeout

I think it's about 7-10min now.

Another request while I'm at it:
"My posts" in forums menu (the one on top) that searches for all post with your username. Bold entries in the search result gives updated threads, so it's easier to follow multiple discussions. I'm not a fan of thread subscription.