Admin CP > vBulletin Options > vBulletin Options > Cookies and HTTP Header Options ('Time-out for Cookie' is what you're looking for)
Just make sure that in your General Settings the Thread/Forum Read Marking Type is not set to 'Inactivity/Cookie Based', or once your user goes off the Who's Online list any unread posts they left will be set to read, which might confuse your visitors!
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