Rear Brake Pad life
On my bikes with Solo use
GS1150 - 19100 miles (changed in a BMW Dealers in Graz, Austria while on tour and the wear limiters had just appeared - in Yorkshire we believe in using every ounce of pad wear)
GS1100 - 22300 miles.
As for fronts - I have never had to replace a set at all and sold my GS1100 with 35,000 miles and my GS1150 with 24,500 miles and at each service,they were never below 2/3rds worn.
With the engine braking of a flat twin,who needs to use the brakes to excess,it's all about observation.