You are basically looking for 600 seconds (10 minutes) in a couple of scripts. Adjust this to the number of seconds that you'd like.
Change this in ubbt.inc.php:
$Outdated = $Outdated - 600;
Change this in online.php:
$Outdated = $html -> get_date() - 600;
Change this in ubbthreads.php:
$LastOn = $date - 600;
For example if you wanted an hour (60 minutes) you would change all the 600's to 3600.
Be sure to also change the reference to "10 minutes" on the /languages/english/online.php to whatever you choose.