I usually 'back into' my flight... meaning depending upon my flight's scheduled departure time, I'll figure out what time boarding starts. Then whether I'm in BF or coach, or if I want to buy something in DF or need to buy food to eat on board (if in coach). And then add in how long it will take to get to EWR (or my int'l destination) from my starting point-- work in Midtown NYC or hotel. (As I alway's only do one carry on and am Plat, I'm not concerned about checking luggage or standing on long check in lines).
Example- Flight leaves EWR at 5:45pm.. I'll assume check in starts at 5pm (I always like to be one of the first to board--or at least that gives me a cushion if I have some delays getting to the airport or thru security). If I want a little time to buy food/DF/visit PC... I'll want to be at EWR at 4:30pm. Thus I leave my office at 3:30 to get the subway to Penn Station for NJ Transit/AirTran to EWR. Same goes for leaving from international cities, I'll factor in public transport to the airport, but will allow a bit more time, if it's someplace I've never been thru before. In general, I'll arrive at an airport 60-90 minutes before a flight.