My rule of thumb would be three hours if there was such a thing as two hour "door to door" air travel. But the minimum for all practical purposes seems to typically be about four or five hours that you have to set aside for even a short hop trip. The only way I'd think you could get under that would be if you had the good fortune to be going airport hotel to airport hotel without needing to park and/or rent a car.

And you're absolutely right....satellite radio and mp3 players have taken some of the boredom out of those long drives.