Maintenance of brake pads is one of the top safety priorities. If one fails to do so, you might be the only one to blame if an accident occurs. It is mandatory to check the pads every start of the ride.
The main classes of brakes are the following two: the rim brakes and the disc brakes. A road bike or a hybrid-build is commonly having rim brakes, either the V-brake or the cantilever. Mountain bikes and others that are built for off-road are commonly installed with disc brakes.
Regardless of type, brakes will wear out if subjected to friction. The more you ride and the harder you brake during, the more it is degraded. The only solution is to replace the brake pads but the question is when.
For rim brake pads, if the rubber surface has become thin and the grooves and teeth are diminishing significantly, then it is time to replace them. You can check their responsiveness by turning the wheel fast, while your bike is stationary, and hit the brakes.
For disc brake pads, they have usually a 3-mm or 4-mm thickness. If the thickness reached closer to 1 mm, then have it replaced.
If the brake pads are new yet you are having a hard time applying the brakes, you can tighten the brake cable.