obviously it would depend uponm your riving habiots & braking use.. Typically it will be a higher performance pad & rotor for higher temps, so wear prob not as good as stock, but basically it really depends too much to really have a life expectancy.. fror street use (no track, etc) mid 20-30k maybe? too hard to say..
realize (you might, im not sure by your wording) that most of the BBK cost is in the calipers and brackets, which do not wear out. You get new pads and rotors if necessary, not another BBK. Of course, these components prob cost a little more than thier OEM counterparts.
My stock 325 brakes were shot at 12k miles. The pads had less than 3mm of material and the rotors were 75% worn.
The UUC VSBs on my car have 30k miles on them (~42k total miles on the car) and I just had to put in pads. The rotors appear to be about 50% life left assuming the slots are cut to the wear limit.
Typical use for both stock and UUC brakes includes auytocrosses, driver schools, and lots of hard street miles. Based on my experience, you should expect at least twice the life out of big brakes assuming you don't buy a kit that comes with undersized Lotus Brembo calipers with tiny pads.
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