Check out php.ini the value set for session.gc_maxlifetime is the ID lifetime in seconds.
I believe the default is 1440 seconds (24 mins)
http://www.php.net/manual/en/session.configuration.php
Edit: As some comments point out, the above is not entirely accurate. A wonderful explanation of why, and how to implement session lifetimes is available here:
How do I expire a PHP session after 30 minutes?