Depends on quality of turntable, tonearm, and vinyl, cleanliness of same, and the setup accuracy of the cartridge and cleanliness (again) of the stylus.
Low end 300 hours. Best conditions, 500-800 hours
Nagaoka MP 300 Cartridge
I was thinking I should know more about the average lifespan in hours of this cartridge so I can start thinking about a replacement. If you can help please let me know.