Grado The Reference 600 hrs old. How many more?

I had over 2,000 hours on a Grado The Reference and it was still sounding good when I sold it and the turntable to a friend who was still quite pleased with it two years later.
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At a guess, your cartridge is still young, not really much beyond "broken in". I have read claims by Grado of "5000 + hours". Why they should be so long-lived is not clear. Nice cartridges, but I personally would not believe any Grado claims, based on my experience trying to get Grado to identify a decent phono stage for a low output Grado Reference Statement cartridge (they refused, and would only push their own low end phono pre)and their quasi-deceptive marketing (relatively few high gain phono preamps are compatible with their low output cartridges, as most are designed for moving coil cartridges, which are completely different.)