Tend to check in in the morning for night flights (which most long-haul ones I take from here are), whether going to work that day or just a day off. That's the nice thing about having an in-town check-in facility. Can then get to the airport an hour or so before the flight leaves, long enough if there's an issue with the train (rare) and long enough for long security lines (again rare if travelling at times not 'popular' with the tour groups).

Otherwise, at destinations without in-town check-in (which are unfortunately most!), then at the airport three hours before. If early, can have a cup of tea; but if late... I can do without that stress.