You vehicle will have a recommended timeframe for when to service your brakes (check the owner's manual for details), but this isn't always reliable due to different conditions putting different levels of strain on your brakes. For instance, if you regularly drive in heavy traffic or hilly terrain your brakes will most likely need servicing more often than drivers who use their brakes less.
Luckily, all vehicles built since the 1970s make certain sounds at the first sign of brake problems, hundreds of miles before the problems become serious. These sounds vary by make and model, but always involve high-pitched squealing that turns into metallic grinding sounds over time.
High-pitched squeaking/chirping when not applying the brakes
Due to the way modern brakes are designed, an occasional squealing sound isn't always an indicator of brake problems - nowadays brake pads are often made from heavy, metallic disks that can produce a squealing sound sporadically, especially in smaller cars. However, if you start to hear a high-pitched sound when you're not depressing the brake pedal (and it ceases or changes in tone when you do apply the brakes), it is an indication that your brake pads are wearing very thin.
The source of this sound is known as a chirper, and it is designed to emit noise once the brake pad has worn down to about 30% of its original thickness. The chirper starts when you still have several thousand miles of wear left in your brake pads, so getting your brakes serviced at this stage is often a fast and cheap process - acting quickly on chirping noises will save you a lot of time and money.
Grinding noises when applying the brakes
A grinding sound is something you should aim to never hear from your brakes. This grinding is usually preceded by the squealing noise mentioned above - it is an indicator of rotor or drum damage caused by the brake pads fully wearing away, and the metal caliper directly scratching against the rotor. This is causing serious damage every time you apply the brake, and can heighten the risk of brake failure in difficult driving conditions that put strain on the brake components.
If you take your vehicle in for servicing as soon as you start to hear this sound, then a mechanic should be able to repair your rotor by "machining" it - evening out its surface to remove any grooves made by the calipers. But leaving it for too long will cause the calipers to scratch deep gouges in the rotor to the point it needs replacing, which will be costly.
If your vehicle starts making strange noises it is always a bad sign, and the noises will usually precede some sort of serious damage. Even if you don't know anything about engines, you can get a feeling for the health of vehicle simply by listening for unusual sounds. A reputable auto repair shop will be happy to offer a diagnosis service for any strange sounds you report, and you will often find that you can avoid any costly repairs or replacements simply by keeping your ears open.
