Frontier Airlines Check In Policy encourages passengers to check in online prior to their flight departure. Online check-in begins 24 hours before departure and closes 45 minutes prior to scheduled flight time for domestic flights and 60 minutes for international flights. Passengers who need assistance may also go to an airport kiosk or ticket counter at least 45 minutes before the scheduled departure time of their domestic flight and one hour prior to an international flight departure. All passengers must be checked in by these times or they will not be able to board the plane.