Although I would not recommend doing this, take a look at http://stackoverflow.com/questions/520237/how-do-i-expire-a-php-session-after-30-minutes?lq=1https://stackoverflow.com/questions/520237/how-do-i-expire-a-php-session-after-30-minutes?lq=1
session.gc_maxlifetime specifies the number of seconds after which data will be seen as 'garbage' and cleaned up. Garbage collection occurs during session start.
You can set this value in either php.ini or your .htaccess file
