The passenger check-in procedure starts 2-3 hours prior to the airplane departure, and it stops, as a rule, 40-45 minutes prior to the airplane departure – both for domestic and international flights. For the air flights of some airlines, the check-in procedure can be stopped 1-1.5 hours prior to the flight departure.
You are supposed to arrive at the airport for the check-in for regular domestic flights 2 hours prior to the flight departure, and for the international flights – 3 hours prior to the time of the flight departure, which is indicated in the ticket (itinerary), for going through checking formalities (the customs, immigration control, veterinary or sanitary control, if necessary, and so forth).
You can also pass the online check-in in advance on the website of the airline, with simultaneous selection of the seat in the airplane passenger compartment. The online registration starts 24 hours prior to the flight departure.