The brakes may begin to squeak at about the 50 percent wear point. This is due to the size and material used for the brake pads and rotors. The brake rotor surfaces become uneven, causing a lip to form at the outer edge. This will generally require replacement of the rotors when the pads are worn (pad life varies depending on driving style and terrain).
Car Problem Reports
Mercedes-Benz S600 Replace Brake Rotors When Pads are Worn and Squeaking
Mercedes-Benz S600 Problem
Average Mileage: 85,000 mi (54,000 mi - 112,000 mi)
squeeks, when replaced had to replace rotors also, cost $1200
Visitor, 2005 Mercedes-Benz S600, 54,000 mi
brake pads and rotors had to be replaced.