Brake pads and brake discs wear is relative to the brake pad choice, brake duct setting, brake bias, ambient temperatures, driving style and ABS settings. Brake disc and pad wear is shown at the end of each driving session, when you return to your strategy setup UI on the "last readings" box.
Additionally brake pads consumption will appear as a red dot in the center of the brake discs heat visualisation in the tyre and brakes HUD, when brake pads are under 10mm thickness.
Make sure that you're choosing the appropriate brake pads for the type of session and/or length of the race you are competing in. See Brake Pads section for more info.
If you feel like or you are noticing brake wear, you will need to brake earlier to compensate for the lower braking performance until you can make a pit stop to change the brakes (if required / desired) or manage until the end of the race.