Airlines such as Frontier and United suggest checking in online anywhere from 24 hours to 60 minutes before a flight's departure time. Doing so allows customers to avoid check-in counter lines and go straight to security once arriving at the airport.
Airlines generally do not allow online check-in any earlier than 24 hours before the flight departs. Travelers can usually check in online even when checking bags. In order to check in online, most airlines require people to enter each passenger's name, exactly as it appears on the reservation, and a reservation code. Airlines such as United allow passengers to skip printing and simply display their boarding passes on a smart phone after completing the online check-in process.