Honda BIG RED 2009 Owner's Manual page 138

Brakes
Brake Pad Wear
Refer to Safety Precautions on page 69.
Brake pad wear will depend upon the severity of usage and driving
conditions. The pads will wear faster in wet or muddy conditions.
Inspect the pads visually during all regular service intervals to determine
the pad wear.
Note that the pads are designed to make a metallic noise during braking
when they reach the wear limit.
Servicing Your Honda 129