Originally Posted by
ua_to_ord
You can standby for the whole day on which you are ticketed. In the extreme, one could standby for an 11:59p departure with a ticket for a 12:00a flight nearly 24 hours earlier. There is no such 3-5 hour rule.
But I think the person has to show up at the airport between 3-5 hours, not stand by for a flight 3-5 hours.