Clutchs on bikes, like cars, last as long as the owner wants them to. There is really no set mileage figure. You'll know when they go bad, the clutch will start slipping (worn friction discs) or not disengaging properly (warped discs). If you have this problem, make sure your hydraulics are working properly before tear down. My old gixxer 1100 had 61,000 miles on it and I think it clutch was original when I replaced it. As for the busa shock, lots of threads in here about it and i'm sure you'll be getting a response