Not sure if it's what you need but try this code for hook
private_start
PHP Code:
$lastdays = 30; // count posts from last 30 days
$lastposts = 10; // need 10 posts from last 30 days to be able to read/send PMs
$time = TIMENOW - (86400 * $lastdays);
$postcount = $vbulletin->db->query_first_slave("SELECT COUNT(*) AS posts
FROM " . TABLE_PREFIX . "post
WHERE userid = " . $vbulletin->userinfo['userid'] . "
AND dateline > " . $time . "
");
if ($postcount['posts'] < $lastposts)
{
print_no_permission();
}