Persistence of session files

Hi all. I'm wondering if anyone can tell me how long temporary session files stay on the server after the session has ended if session_destroy() is not called. I'm curious as part of the CMS I've been writing for the past year is a class which dynamically generates and validates all forms in the CMS. This class relies heavily on sessions so I've implemented a method which is called upon the execution of every page within the CMS, and which checks for expired forms and deletes them from the session. This is proving to be very effective in minimizing the overall size of the session while still having the user work with very large amounts of session data. However if the user just closes the browser instead of logging out the final cleanup isn't called and there can be some large session files left on the server. What I'm curious about is how long these last before they are somehow deleted? Is there some way I can control this? Cheers and TIA, Pablo.