All of the bikes I work on have some kind of braking system – in fact, except for some fixed-gear bikes, pretty much all the bikes I’ve ever seen have some kind of braking system.
Most older bikes have some kind of rim brake, whether calipers, cantilevers, U- or V-brakes. The pads on these brakes last long, but still wear. Do yourself a favor and check out the pads on your rim brakes. There are often slots cut into the pads to hep remove moisture and dirt. I don’t think it’s a good idea to wait until you’re down past these cuts to change the pads – if your pads look worn, change ‘em. Also, the pad material often hardens and gets less responsive over time. Even if there appears to be substantial material left, it might be a good idea to change your brake pads – and for rim-brake bikes, pads are an inexpensive upgrade. If you have carbon fiber wheels, you’ll want pads that are specifically formulated to work with ‘em; for ally wheels, your choices are more broad. I have my favorites, as do many riders, and there are good reasons why people choose the pads they like (not always just because of price).
(For rim brakes, wheel rims will also wear. I’ve seen some bikes where the rims were so worn, they split. It’s a good idea to examine your wheels periodically, as well. One tell-tale sign that rims might be wearing out is a concave feel on the braking surface.)
For disk brake bikes, you should be checking your pads more frequently. I’ve seen estimates of disk brake pads lasting as little as 500 miles, and as much as 1500, based on material and rider use. This is far less than the life of rim brake pads, and, I think, far less than most riders of disk brake bikes expect. Pad prices vary widely, depending on material, the brakes they fit, and other criteria, and riders should balance considerations like expected life and performance when choosing replacement pads. If pads are allowed to wear too much, the metal backing can damage the rotors, which will then also need replacement. Pads are often sold with 3mm thickness of braking material, and manufacturers suggest replacement when the material is worn to a thickness of 1.5mm to 1.8mm, depending on the manufacturer.
Rotor prices also vary, but rotors are not generally far more expensive than pads. One manufacturer sells rotors that are 1.8mm thick, and recommends replacement when they are worn down to 1.5mm, and this is the standard I use at Ramblin’ Wrench when I measure the thickness of your rotors. Rotor thickness should be checked with a caliper.
Different riders use their brakes differently. Most of the braking is done by the front wheel, so many riders use primarily the front brake; others use primarily the rear, or try to balance their brake use. Thus, you may wear out one brake earlier than the other. I don’t use disk brakes, but if I did, I’d keep a set of pads and a rotor in the garage. With current bike part supply chain problems, I’d hate to have to wait for the container that had the part I needed, if it were unavailable during riding season!