If you mean the brake pads that are used in the disc brakes then if they need replaced you should notice that your bake pedal is going further to the floor than it used too, and you might also hear a squeaking or squealing noise that is caused by a little piece of metal that makes contact with your brakes rotor when the brake pad is worn down.
Brake pads normally need replaced around 70,000 miles but they can last much longer if you don't use your brakes as often such as in stop and go traffic.
Brake Rotors should be replaced if you start noticing cracks in the rotors or grooves in the rotors or if you notice the brake rotors getting thin.
Brake rotors last on average of 70,000 miles as well and I've seen some last even longer than that. But if your having brake pads replaced it's sometimes a good idea to just go ahead and have new brake rotors put on if you have the money to do so.
Having good brakes is very important to yourself and your family.