Pad wear is largely dependent on driving conditions and habits. I examine my pads to see when they are getting thin to save the sensor. Otherwise, wait until you get an alert and repace the pads and sensors. By the way, I"ve got 75K on my Axxis Deluxe pads and they still look good.

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2013 X5 3.5i M Sport
2004 M3 cabriolet SMG