Always verify with your air carrier to determine how long in advance of your flight time you must check in. You should arrive at the airport in plenty of time to check in, clear security and reach the departure gate. Check in as soon as you arrive at the airport and obtain your boarding pass.
Check-in times vary from carrier to carrier. Be sure to comply with your carrier's check-in time limit or you may be refused transportation on your flight with no liability on the part of the carrier.