When do I have to be present at the airport?

Exact details on the opening and closing hours of the check-in are usually available on the airline's website or in the documents that were sent together with your ticket.  If you booked your flight through a travel agent, they will normally be able to give you this information.
 
In general passengers are advised to arrive at check-in two hours ahead of their scheduled departure time for intra-European flights. This is a recommendation, not an obligation.
 
Passengers are themselves responsible for arriving on time at the check-in desks and the boarding gate even if they are late owing to circumstances beyond their control (car broke down, public transport delays, problems with identity papers,...).