What is the average lifespan of a phono cartridge?

Hard to say, because there are differences from Manufacturer to Manufacturer.
I had a - heavy - Takeda Miyabi which was still strong after 8 years or regular
use (a few times per week), had a Shelter which was down after 1.5 years.

you don't see it as a red flag that used cartridges were always used for about "200 hours?" Many here will buy just about anything used, except a cartridge.

New phono cartridge is almost like new guitar strings. Unfortunately longer service makes sound more dull. In most cases I tend to keep it for 600 hours before purchasing or trading in to a new one.