The easiest way to know when to replace the pads is when the brake pad warning light illuminates on the dash. Each brake pad has a "sensor" that consists of an electrical loop of wire that is inserted into the brake pad and when the pad wears down far enough the loop is broken (which breaks the circuit) and the light on the dash turns on.
Alternatively, you can manually inspect the pads and replace them just before they wear down to the pad sensor hole that is molded into the edge of the brake pad. This will also save the cost of a new brake pad sensor.
Also, be sure to measure the thickness of the rotors to be sure that they are still within allowable thickness.
Brake pads are a fairly straight forward DIY and there is a lot of experience and help available here on the Forum.
Here you can see the pad sensor holes on the edge of this new set of pads...
Here is a completely worn set of pads - don't wait this long!
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