I'd say you will wipe off the rear within 10k and the front would last until 15k. Of course it depends on the way you drive and the roads you drive on too.
2000 Z3 2.8 TiAg (retired)
2002 330i Orient Blue (retired)
2005 330Ci TiAg (retired)
2009 Z4 30i Space Gray (retired)
2011 Z4 35is Space Gray
2011 535i GT xDrive Space Gray