The official answer is 3 hours, and that's what the airlines advise. Given the number of posts we see where people say what a nightmare the airport happened to be on a given day, or the traffic getting to the airport was terrible, etc., it just makes it difficult to try to purposely cut it close. So I always estimate a reasonable drive time to the airport (factoring in if I need to return a car) and aim to be there 3 hours ahead. If the traffic or airport is abnormally bad, it gives me a buffer and I have some wiggle room. 9 times out of 10 I don't need all the time, but every now and then I'm glad I have it. Better to have to kill an additional hour at the airport than be all stressed and end up a minute late.