There are different time restrictions for check in and boarding requirements of United Airlines and United Express depending on the airport from which the flights are originating. In general, United does not accept baggage for check in for more than 4 hours from the time of departure of flights. The check in is available from 24 hours prior to scheduled departure of flights.
For the flights within US, Puerto Rico, Guam and US Virgin Islands, the minimum check in time is 30 minutes before departure of flight. The time of check in is, generally, 45 minutes except for ST. Thomas at US Virgin Island and Guam where the check in time starts 60 minutes before scheduled departure. The time for presenting oneself at the boarding gate, in general, is 15 minutes from departure and 30 minutes in case of Guam.
For most of the international flights operated by United Airlines and the UnitedExpress, the general check in time is 60 minutes before departure for all the airports. Check in time is 90 minutes for Federated States of Micronesia, Fiji, Marshall Islands and Palau and for Lagos in Nigeria. It is 75 minutes for Dublin in Ireland and Lima in Peru. You are required to be at the boarding gate at least 30 minutes before the scheduled departure in general for all airports. However, boarding time is 60 minutes for Lagos in Nigeria and for Federated states of Micronesia, Fiji, Marshall Islands and Palau. This is 30 minutes for Ireland in Dublin and Lima in Peru.