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 SL500 Replace Brake Rotors When Pads are Worn and Squeaking
Mercedes-Benz SL500 Problem
Average Mileage: 25,500 mi (19,000 mi - 32,000 mi)
Visitor, 1994 Mercedes-Benz SL500, 32,000 mi
Rear brakes squeelloudly upon backing. They are quiet otherwise after driving.