We use 150 ft for the first round (we assume that if you fall somewhere in the middle of the round, that you only fall for 3 seconds before your next action). All rounds after that is 1,000 ft, which is a simplification, but is easily explained away by having .98 gravity and slightly thicker atmosphere (owing to the fact that atmosphere in a D&D world does not thin as you get higher... it reaches the plane of elemental air).
150/1000/1000 comes close enough to reality that we can overlook the weirdness of falling like a soft, light pillow.
We treat Feather Fall as an AOO in response to "Ah, I'm falling!"