In general, we suggest being at the airport at least two hours in advance for national and international flights and three hours in advance for intercontinental flights; however, there are several factors that should be taken into consideration.
Check-in times may vary depending on the airline, the route, the type of airport and, of course, whether there is a queue or not. In fact, check-in counters almost always have long queues.
Then consider the time needed for security checks which, also in this case, can depend on multiple factors (seasonality, airport organisation, etc.).
We therefore recommend always checking in online; in fact, if you're only travelling with hand luggage, you'll be able to go straight to the security checks; if you need to check baggage, many airports have desks dedicated to passengers who have checked in online and only need to check their baggage.
If, on the other hand, you haven't checked in online and therefore don't have a boarding pass, or you need to check baggage, consider that airlines usually open check-in counters for national and international flights two hours before departure; on the other hand, check-in generally begins three hours before departure for international flights.