First and foremost you never want to wait too long to have your brake pads replaced. Repeatedly using your brakes over thousand of miles will eventually wear down your brake pads and they will need to be replaced. How can you tell when it is time to get a brake job?
On most cars it is not to hard to figure out when your car is ready for a brake job. If your car has disc brakes then they will come with what is called a wear indicator. It is a small metal piece embedded into your brake pads. When the pads have worn to a certain point the metal will come in contact with your rotor and the wear indicator will make a squealing noise. This noise should be very noticeable and you should have your brakes checked by a professional mechanic immediately.
The brake pads on your car should go for tens of thousands of miles. This will heavily depend on the way you drive your car. If you ride the breaks a lot or make frequent stops on a regular basis then your brakes are going to wear down at a faster rate. The bottom line is the more you use your brakes the faster they are going to wear. If you feel your brakes are reacting differently than normal, then it is time to have them inspected before further damage is done. Putting off having your brakes checked can be a very costly repair.