When to chang?

Answer 1

The brake pads only cost $12 and they are very easy to install, so the minute you hear a strange noise when you apply the brakes, you should change them to prevent damage to your rotors.

You can also check the wear on your break pads to see how much padding is left. If they look thin, then change them.

There are more signs to a brake problem. If you feel your brake pedal kinda go up and down (pulsating) your rotors need to be replaced. Soft or spongy brake pedal again could be brake pads to thin or air in the brakes, etc. While braking your car is being pulled to the left or right this is a sign of one or more calipers starting to seize.