I've been watching my rear brake pads getting thinner for the past year, and so, I decided I should finally change them before the metal-on-metal grinding begins. I could have changed them sooner, as the previous owner of my DR included a new set of brake pads when I bought the bike. The new pads have just been lying around in my tickle trunk down in the basement. I just figured I should try to get as much mileage as I can before tossing them. So, with 22,090 Kms on the odometer, the original pads are pretty much worn down to almost nothing. I'm sure it would have only been another week or so before the brake rotor would be scored.
I'll have to do the front ones soon, but the originals still have a few miles left.