[Lynne]

You may create a plugin to change it. I did something similar only for a certain forum I increased the time.

PHP Code:

if (is_member_of($vbulletin->userinfo, x,y,z))  
{ 
    $vbulletin->options['edittimelimit'] = 'xx'; 
} 


I used the fetch_foruminfo location since mine was forum specific (my if statement was in regards to the forumid, not the usergroup like I put above). So, try some different locations to see what works.