When to replace your brake components
It is important to know that disc rotors and brake-pads are both disposable brake components, they should be replaced when needed as they ensure your car stops. How often you replace these components can depend on various factors such as your driving style, the material/composition used in the brakes and how well maintained your brake system is.
Disc Brake Rotors
An easy way to check if your rotors need replacing is to check the thickness of the braking surface. Generally, more thickness means more life left in your rotor (assuming there is no other issues with your rotor). Manufacturers will often state the Minimum Thickness of their rotors, rotors will require a minimum thickness to be safely used with no loss in braking power. If it is lower than the minimum thickness then you must replace the rotors or risk extremely poor braking.
Brake-pads are made out of different material compositions, because of this they can vary in brake wear rate and amount of brake dust. Generally, aggressive brake-pads such was ones used for track use or heavy street use will cause rotors to wear out faster as they are braking harder and generate brake-dust. Your mileage will vary depending on the type of brake-pad you use and the other factors as mentioned previously.
Some brake-pads will now also have brake-wear indicators in-built , these are a metal component that will cause a squealing sound when it comes into contact with the disc rotor. When it comes into contact with the rotor it means the brake-pad material is running thin and needs replacing. Also available are electronic brake-pad sensors, these are disposable sensors that instead of making audible sound will light-up the warning sign in the drivers dashboard.
Pictured above: Left brake-pad has lots of remaining material, Right brake-pad has been completely worn out and needs replacement.