No im pretty sure its not il the 31st minute like stated above. Your cookies and such leave you logged in since they are always like there/on. So the site we can say 'removes them' or "overrides them" after 31 minutes.
Well, it's used for setting new threads. At times, I have exceeded this timeout while reading the material and references on one page of a thread (though this is increasingly a rarity...). This, of course, led to me losing my place in the balance of the new threads.