Not meaning to be a wise guy, the answer is when it needs it. There only two reasons to replace a rotor, either it's warped (you'll feel some come and go surging when you step on the brakes) of it's been worn below the allowable thickness.
If its not warped and not seriously grooved then just put on the new pads and they'll fit themselves to the rotor. If its seriously grooved then you can have it resurfaced if there's sufficient thickness remaining (your machine shop will check for this when you take it in) or replace it.
The rust you see is on the rotor hub and has no bearing on brake performance. If you don't like it, try painting it with hi-temp flat black paint.
Last edited by Carpe Diem; 03-14-04 at 03:15 PM.