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.
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.