If the pads are worn beyond specifications in conjunction with worn rotors then there is the possibility of the piston not being long enough to accomodate the wear and thus the piston can cock sideways in its bore and possibly leak or cause damage.
Minumum thickness of the pads is 2mm of friction material left. A nickel is 1.95 mm thick, so this makes an easy way to tell how much is left. If the pads are thicker than a nickel, then you have time, if they are thinner than a nickel, then they should be replaced soon.
The lights on these cars are rigged to give ample warning to when the pads need to be replaced. On my car, the light comes on with 3 mm of pad left.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record