Brake pad wear comes down to one measurement: how much friction material is left. Knowing the thickness numbers - in millimeters - takes the guesswork out of when to replace pads. Here is the simple scale from new to worn out.
New pad thickness
A brand-new brake pad has roughly 10 to 12mm of friction material on the backing plate (some are a bit more). That full thickness is your starting point - everything from there is wear. The pads do not all wear at the same rate; fronts wear faster than rears - see why rear pads last longer.
The numbers that matter
As a practical scale: around 6-7mm is half-worn and fine; at about 3-4mm start planning replacement; at 3mm or less it is time to replace now. Most wear-indicator tabs are set to squeal around 3mm - see the squealer tab. Do not run pads below about 2-3mm; past that you risk metal-on-metal and rotor damage - see when to replace brake pads.
Why not run them to zero
Waiting until the pad is gone means the steel backing plate grinds the rotor - see why brakes grind - which ruins the rotor and can cut braking power dangerously. Replacing at 3mm costs pads; running to zero costs pads AND rotors, and risks safety.
How to check
You can eyeball pad thickness through the wheel spokes against the rotor, or measure precisely with a gauge when the wheel is off. Replace pads as an axle set once any pad hits the limit, and fit fresh hardware with your new brake pads.