Is there a standard measurement at which the pads should be replaced , or does it vary by vehicle ? My wife's CX-9 has 100k on the original pads . I had it in the shop for a brake booster replacement and the tech said that the pads were thin but not metal on metal yet . He said if we ever hear it squeal , get it in the shop immediately . I'm just curious about the actual measurement because I see it mentioned sometimes .