UA policy (which is in line with DOT policy) is full refund within 24 hours of purchase (there are some old reports of this occurring a few minutes beyond 24 hour deadline). UA also seems to go above DOT policy because DOT doesn’t require a carrier to refund within 24 hours of purchase if travel starts 7 days or sooner, UA stated policy follows those guidelines, but all real-life reports I’ve seen in here indicate they allow the refunds within 7 days of travel.

It’s been a long time since UA offered refunds until midnight next day (in those days, you could book a ticket at 12:01am and have almost 48 hours…) - probably before the 2012 merger. Delta definitely allowed this much later than that, though even I don’t think they do this anymore. TAs each have their own policy…some do allow until midnight next day.