Changed all four of mine for non runflats and had 6mm left on the fronts with backs worn fully @ 18k
old car 320cd msport got through all four @ 16k, and pads and disks all round @19K
depends on roads you use and how you drive, changed my route to work when I bought the Z4 to use more A roads, not so much fun maybe but fuel economy is much better, and tyres and brakes last so much longer.