No exact time definition exists which clearly states how long before departure a passenger is obliged to be at the airport.
Only the time of check-in close up is clearly determined. At this time the last passenger has to be accomplished. In case when only one check-in desk is open for his flight and a long queue of passengers is waiting for accomplishment in order to minimise the risk we STRONGLY RECOMMEND to come to the airport:
2 hours before departure for a short and medium long fligh:
3 hours before departure for a long flight
It is much more probable that you are accomplished in time.