Honestly, there is no reason why someone should be penalized for leaving a dungeon or trial after 30 minutes have passed. That's long enough to give the duty a good try, and at that point it should be clear that the party either does or doesn't have the ability to clear whatever it is they're in. There's a difference between "helping" and "beating your head against a wall" and sometimes you really do get stucks with groups that are the latter.