If no one gives you an answer, just take a pocket ruler with you to your local Harley dealership and measure the friction material on a bike on the showroom floor. Just measure the friction material, not the metal backing.
Wear rates will vary from bike to bike and from rider to rider, and often even from one set of pads to the next. Harley's minimum friction material thickness spec is .040" (1 mm). IMHO that is cutting it way too close, my personal absolute minimum thickness is about 3/32" (0.090"). And don't just look at one pad, look at all of them, inner and outer and all three sets. It's kind of rare for all three sets of pads to wear at the exact same rate. I think I know what you're trying to do, just schedule to replace them all at X miles with other maintenance. You can do it that way, but there is no way you can accurately predict how each pad will wear over time so to be safe you would most likely be changing them too often. Conditions change, the pads themselves change slightly from one to the next, and how you actually ride can change. IMHO a better idea is to spend a few minutes at every service interval visually checking the pads. It would also be a great time to do an in-depth tire check, looking closely for damage, cracking, and abnormal tread wear. But that's just my opinion of course.
Jerry