Are you ok with editing files? Navigate to yourforum/System/Session/Store folder and open the Database.php file. There find this code:
array( 's.running_time>?', \IPS\DateTime::create()->sub( new \DateInterval( 'PT30M' ) )->getTimeStamp() ),
array( 'core_sessions.running_time>' . \IPS\DateTime::create()->sub( new \DateInterval( 'PT30M' ) )->getTimeStamp() ),
And change 30, which is the number of minutes for the online session time. I have not tested this, but I think that it shoud work. Hope it helps.