In the same boat as you ... I believe it is 2mm worth of wear and then the friction rings should be replaced. I have been using ST pads but I am going to run RS29's at VIR in a couple weeks. I am going to measure mine but I doubt they are toast after just a few track days.
2013 E92 M3 with ESS VT2-585 SC, StopTech BBK, ESS Full Exhaust, and a bunch of carbon fiber ...