I have a 1970 bike that I bought used in the early eighties with obviously quite a few miles with what I am reasonably certain is its original C in a diamond NR headset. For around ten years that bike was my only transport and I rode it everywhere piling up many, many thousands of miles including a fair bit of off road duty. It's still my daily rider and I've had the headset apart I think twice in those twenty odd years. All the parts including the bearings are the same ones I bought it with and it still feels fine.
I'm left wondering what the hell you have to do to damage or wear out a properly adjusted NR headset because it seems damn near impossible to me in one lifetime.