20k - 30k out of pads. Much longer out a set of rotors if you don't let the pads get down to the point where they grind grooves into them. Or if they're warped.
A lot of people, and a lot of brake shops would have you believe that the rotors need to be turned every time the pads are changed. This is absolutely not true.
Only reason rotors needs to be turned:
1. Gouges deeper than a specified thickness.
2. Warpage. Even then I always replace - if they're warped, they'll warp again.
3. Under minimum thickness.
Most of the automakers are now recommending just changing pads and leaving the rotors if none of the above is true.