Question for all you New Yorkers out there: How much time do you allow to get to the airport?

I guess others can chime in with their answers (other cities). Anyway, I was just wondering how long you estimate it takes to get from someplace in Manhattan (Chelsea, UES, Midtown, whatever) to either LGA or JFK. You can ume that you can print boarding pes ahead of time and will be only carrying on luggage. The thing is: I want to sleep in, but I don't want to miss my 6:30 am flight. How much time should I allow. What's the time difference between taxi and subway/bus? If you would rather, you can pretend it's a different time of day, but early AM is usually quite busy, right?