This one varies too greatly to give a useful number but is the easy to check on your own.
Turn your handlebar all the way to one side and look at the back of the caliper.
Never let the metal shims with the brake pin touch the rotors.
In the red circle, is an indent in the brake pads. Check on the wear marker.
If the pad is down to the wear marker you should replace them.
The rear brake is harder to check on because it's practically underneath the bike but the same concept applies. There is only one small rotor in the rear.
If your rear is significantly diminishing before your front, you may want to adjust your riding style so you brake less with the rear and more with the front. I save the rear for slow traffic, hill stops, and parking lots, u-turn stability : )