ValekS, вот, кажется получилось:
$my = mysql_result(mysql_query("SELECT `time` FROM `forum` WHERE `type`='m' AND `refid`='" . $res['id'] . "' AND `user_id`='".$user_id."' ORDER BY `time` DESC LIMIT 1"), 0);
$not_my = mysql_result(mysql_query("SELECT COUNT(*) `id` FROM `forum` WHERE `type`='m' AND `refid`='" . $res['id'] . "' AND `time`>'".$my."' AND `user_id`!='".$user_id."' ORDER BY `time`"), 0);