I don't have a percentage, but seems to be pretty consistent for all the Benz' I've worked on... the light always comes on right after you pass the point that those little brake wear sensors will come out without breaking. I could usually get at least 2 of them out on each brake job, but always ordered 4... Most people 'brake on through' the sensors before they do anything about it. Unless the style has changed in the last couple years, the wearing part of the sensor is *maybe* the diameter of a wire coat-hanger. So, that's the thickness of the brake pad stopping material when the light comes on. You still have a good thick bit of metal once you're past the pad lining... and then the piston of the caliper after that... not to worry.