There doesn't seem to be any real pattern to the problem.
I've had a few recently that have needed replacing. 748R a little over a year old, 6k. 748BP, '99 ish 4.5k miles, Rockhoppers ST4 can't remember details (sorry Paul), yet my own bike '98 748, has 17k miles on it and hasn't had one yet.........ever.
It's about time that it was sorted IMO. There have been several part number changes and the only thing that seems to change is the price..........it goes up.
I'm starting to look around at some hard chroming companies and see if they can be refurbished and have proper chrome plated on them. It's not rocket science. It just needs to be done properly to an agreed standard.
rant over