When you look at fresh pads, they are only 2-3mm of media on a backing plate. They should perform fine throughout the thickness of that media, provided there is no contaminants present.
My preference is to replace em when the media has only .5-.75mm remaining.
Another item to look at is rotor thickness. Metallic and ceramic pads tend to annihilate rotors. Same happens on cars! Organic pads tend to make rotors last longer.
I replaced rotors and pads on a friends bike recently. The rotors were paper thin and sharp as a fresh scalpel. Those were well below minimum thickness for safe operation.
Inspect the calipers, rotors, hoses and master cylinders at the same time to be sure you have a viable, safe brake system.