Dear Annonymous,
There is a small device on most brake pads commonly called a " Screecher" because of the high-pitched squeal it produces. It is a small metal tang built in to the pad itself. As the pad wears down the skreecher comes into contact with the surface of the rotor thereby telling you that it is now time to change the pads. With diligent attention to the mantainence aspect of car ownership, you must observe by yourself, you may not have to replace the rotors at all this time or next. The compounds used in the pads are softer than the steel rotors and wear or "Marry" together and keep the excessive wear of the rotor to a minimum, thereby allowing you to use the rotor more than once, even twice. I have done it. So, pay close attention to the tales your car tells you, and you will be pleasently suprised at how much you can save by being a good car owner.
Boris427