I think I remember reading somewhere it's around 20K miles or so, but again that depends how you drive. I guess the only way to really tell is to take the wheels off and inspect them if you are concerned.
This is when to change the pads. The top one is an OEM Brembo pad that has been overheated and came loose from the backing plate. Just below the caliper (standing on edge) is a PFC pad that DIDN'T fall apart at the track.