The cost of skimming the discs makes it a bit of a waste of time tbh ie you may as well start thinking about new. As soon as you're seeing a lip of a couple of mm in the discs (min 22mm front, 12mm rear)) then it's time to start thinking about replacing. If you let the disc wear the pistons come further and further out to engage the brakes, and then corrode, and then it will be a caliper refurb job on top of the disc change. Since it's so easy to change the discs on a RR (as long as the PO didn't torque the retaining screw!!) then it's best to stay on top of them