if (isset($foruminfo))
{
global $vbulletin, $foruminfo;
$edittimes =array(c1=>t1, c2=>t2...);
$parents = array_intersect(explode(',',$foruminfo['parentlist']), array_keys($edittimes));
if (($parent = reset($parents)) !== FALSE)
{
$vbulletin->options['edittimelimit'] = $edittimes[$parent];
}
}
where you'd change c1, c2, etc to the category ids, and t1, t2, etc to the corresponding times in minutes.
I haven't test it except to see that there aren't any typos. I changed the 'if' condition because it looks like edittimelimit is used in some other places.