Check-in for air travel differs depending on your starting destination, your final destination, the airline you are flying and your travel day and time. Typically you should arrive at least two hours early for international flights and one hour early for domestic flights. However, specific details will help you decide exactly how early to arrive.
The website of the airline you are traveling will offer advice on when to arrive at the airport. You still have to account for other factors, but it will help you get a general idea. The American Airlines website recommends arriving at least 60 minutes early for domestic flights and two hours early for international flights. You must check in at least 30 minutes before a domestic flight and 60 minutes for international flights; otherwise, American Airlines has the right to cancel your ticket. Delta Airlines has the same travel guidelines. The same restrictions and recommendations are posted on the United Airlines website; however, your international ticket will not be canceled until 45 minutes before your flight.
If you are changing airlines mid-trip, make sure to call the airline ahead of time to make special arrangements. If you arrive at your layover and have not checked in for your international flight, you may find that your ticket has been canceled.
Allot at least an extra half-hour if you are checking baggage, even if you can choose this option while doing online check-in. You will still have to visit the baggage counter, which requires additional time.
Busy Days and Times
Depending on which airport you are flying from, certain days of the week and times of the day will be busier than others. According to United Airlines' website, the busiest days of the week and times of day at many airports are Saturday, Sunday, Friday and Monday. Peak hours are generally first thing in the morning (6 a.m. to 9 a.m.) and early afternoon (noon to 3 p.m.).
Busy passenger airports take longer for check-in due to longer lines at check-in and airport security. According to Meterdown, the world’s busiest airports are (in order) Atlanta, Chicago O'Hare, London Heathrow, Tokyo and Beijing.