Well, they should be serviced before they won't stop your vehicle anymore, that's for sure!
ABOUT YOUR BRAKING SYSTEM:
Today's braking systems are composed of a number of parts that all work together to ensure that your vehicle stops properly.
Most brake systems work this way: when you depress the brake pedal, you are telling the master cylinder to compress brake fluid, which in turn sends hydraulic pressure through brake lines to each set of brakes at the wheels, and that activates your vehicle's brake pads (or shoes). These pads (or shoes) then make contact with spinning rotors (or drums), slowing down or stopping the vehicle. There are more complicated systems that use sensors to activate the braking system, but the effect (stopping the vehicle) is the same.
So, how often you need to get brake service depends on the type of driving you do! City drivers need it more often than someone who does most of their driving on an interstate.
The parts that make up your vehicle's brake system wear down over time and eventually need to be replaced. The most common brake repair is to replace your brake pads (or shoes). Regardless of what they are made of, brake pads wear each time you depress the brake pedal, that's how the create the friction that stops the vehicle.If your brake pads are allowed to wear down too much without regular replacement, the metal backing that holds the pads will make contact with the brake's metal rotors (or drums), which can produce not only a safety issue of braking effectiveness, but a situation where the increased wear on the rotors/drums means they need to be replaced as a result of the prolonged contact of metal parts without the pad as a buffer.