When a drive belt should be replaced in your car?

Your engine has a number of mechanical accessories attached to it, such as an alternator, water pump, power steering pump and an air conditioner compressor. A drive belt is the part that drives all these accessories. The belt itself is driven by a pulley attached to the engine crankshaft.
Drive Belt Drive belt
Most cars have one or two drive belts. When a car has only one drive belt, it might be called a serpentine belt. A drive belt is made of a durable rubber-based material.  In most cars, a drive belt is replaced between 40,000 and 70,000 miles, however, we have seen cases where a drive belt lasted over 100k miles.

What makes the belt wear out sooner? Water, road dirt, excessive heat and oil leaks can cause the belt to deteriorate prematurely. For example, if the protective shield underneath the engine (engine undercover) is broken, the belt will need to be replaced sooner. Oil leaks can also damage the drive belt. In this photo, the belt is falling apart because it's soaked in oil leaking from the engine valve cover gasket. A worn-out drive belt tensioner can also cause the belt to break earlier.
Eventually, a worn drive belt will break. See the photo below. A broken belt can leave you stranded by the side of the road. This is because the engine will overheat if the water pump is not running. Also, if your car has a hydraulic power steering, you will lose the steering assist, as the power steering pump will be disabled. In addition, the alternator will stop supplying electric power that is needed to recharge the battery and run the vehicle electric systems.
Broken drive belt Broken drive belt
When should the serpentine belt be replaced? Most car manufacturers don't specify the exact mileage, but recommend to have the belt inspected during your regular maintenance visits.

How can you tell if the serpentine belt needs replacement? Your mechanic will look for cuts, cracks, missing chunks, glazing and other signs of wear; usually it's fairly easy to see. For example, the belt in the next photo shows cracks and missing chunks of the grooves on the ribbed side. The drive belt in the photo below has minor cracks. It's also recommended to be replaced.
Worn out drive belt This belt is worn out
The belt may also need to be replaced if it makes chirping or squeaking noises or if it's stretched. It is also a good idea to replace an old drive belt when an alternator or any other component driven by the drive belt is replaced, or when you prepare your car for a long road trip. To work properly, a drive belt must be under proper tension. Most newer cars have an automatic spring-loaded or hydraulic belt tensioner that doesn't need to be adjusted.
Cracked drive belt Cracked drive belt
In older cars, the belt tension had to be adjusted manually in regular intervals. One of the symptoms of a loose drive belt is a loud squealing or screeching noise that lasts for a few seconds after the engine is started. Often, drive belts are replaced as a kit with the new tensioner and other hardware.

Replacing the serpentine belt may cost from $75 to $250, depending on a car. Is it easy to replace the serpentine belt as a DIY project? If you have sufficient mechanical skills, proper tools and the belt routing diagram, it's not very difficult. Your parts store may help you with the routing diagram and tools.