You can typically join a standby list no more than 24 hours before your original flight. Most major airlines, such as United and American, allow passengers to request a "same-day standby" through their mobile app, website, or at an airport kiosk starting the day before travel. To be eligible for free standby, your new requested flight must generally have the same origin and destination as your original ticket and must depart on the same calendar day. The actual seat assignments for standby passengers are usually made shortly before departure—typically 30 minutes before for domestic flights and 60 minutes before for international routes—once the airline confirms how many checked-in passengers have failed to show up. It is important to note that standby is a "waitlist" and not a guarantee; priority is given based on elite status, fare class, and the time the request was made. If you absolutely need a specific earlier flight, a "same-day confirmed" change is a safer (though often paid) alternative that secures your seat immediately.