See the following thread located a few pages back here in the XC90 section. It talks about rotor part numbers and sizing. Your dealer or a good parts supplier should also be able to provide you with thepart numbers / parts you need after you tell them the configuration of your XC90:
We got similar life out of our front rotors, and had the dealer replace them at around 55K miles as I recall. We did both front pads and rotors as a set, incidentally, and is recommended. The rear pads/rotors are factory original at 67K miles.