I am only familiar with one limit, the minimum thickness that is stamped on the disk. Not saying BMW doesn't have two numbers, but I don't have a manual or service bulletin to know that.

I personally never trust the assumed wear rate, and so if they are anywhere near the minimum thickness or will be by the time the new pads are projected to wear out, I just replace the rotors. I usually just replace the rotors anyway, to get the benefit of a better friction surface and reduce the chance of any pad noise issues, since turning rotors went out of fashion.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue