Min permissible thickess of the pads is 2 mm not including the backing plate. Wear sensor should activate with at least 3 mm remaining. You should be able to look through the wheel with a flashlight and see how thick the pads are.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record