Brake life really varies a lot by driving conditions and driving style. If you have a smooth, non-aggressive driver who anticipates braking requirements and does mostly level highway driving, you have the best case. If you have a jerky, aggressive driver who never anticipates braking requirements and drives exclusively in city traffic in a place like San Francisco, you have the worst case.

As a result, it is almost impossible to provide an accurate "average" brake life.

I have not had a BMW long enough to need a brake job, but typically in the past I have always gotten 50K-75K before my first brake job, while some with identical vehicles drvien by others have needed new brakes after 15K, e.g., a Ford Focus.