My 3 coin rule
If your brake light is on and you have a lip whose thickness is no more than that a "Dime" - I might used them again.
"Pennys" - its about time to change them
"Nickel" - Regardless brake light or no hurry up and change em'
For you international guys. Not sure about the thicknesses of your coins and the corresponding equivalent in your "funny money"
** (You measure it by placing the coin flat on the smooth part of the rotor and slide out toward the edge till you get to the "lip". Then run your finger over the lip and the coin. Which ever coin is closest in thickness to the lip the above applies)