I rotated purely so I didn't have to buy any new tyres before I got my new R. 8/9k miles is probably about right for all tyres being done in at the same time, I waited to 11k miles and at 15k miles now my rears (which were my fronts) are probably around 2.5mm, should last until April/May for the R, but the fronts (were the rears) are still a healthy 5.5mm.