My wife, daughter, niece, and I have an upcoming UA DEN-HNL flight. We are all on one reservation, purchased by me as refundable Y fares.

There is a chance my niece might not be able to make the trip. Is there a time limit before check-in and/or departure that I need to follow in order to get a full refund for my niece's ticket if we learn that she is not coming? I know it has to be before departure, but I don't know how long before departure.

Thank you!