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.
Current: 2014 F10 M5
Previous: 2015 F80 M3, 2013 E92 M3, 2013 F10 M5, 2009 E90 M3, 1998 E36 M3