Mine were replaced at 28K miles under warranty. 46K on the clock now and still hitting all ignition targets set and running better than ever.
I'll probably let BMW change them around 58K under the extended maintenance contract I purchased. Along with rotors, pads, brake fluid, oil, micro-filter, tranny oil, diff oil, and clutch if I can be convincing enough
And yeah... tires too somewhere in there. Mine took a beating at the track a few days ago.