What is the average lifespan of a phono cartridge?

It depends on the stylus profile: Conical has very short life up to 300hrs, MicroRidge has the longes life up to 2000 hrs!

I see ads for used cartridges stating that its barely broken in at 200 hours

This is not true, even for Elliptical stylus is almost half life!
You don’t have to worry about Shibata, MicroLine, LineContact, Stereohedron ... if you’re sure that it was in use just for 200 hrs, but normally an audiogon seller who stated 200 hrs for each of his secondhand cartridges is a cheater, he has no idea because he’s not a first owner and does not documented an hrs of use. Some of his vintage carts are already worn out and inspection by SoundSmith or any other Re-tipper is a must have! Especially if one particular sample does not sound as good as another newer sample of the same cart. 

Well, i am not afraid of used vinyl and my interest is original pressings from the 70's. The benefit of playin used vinyl with Shibata, LineContact, MicroRidge, Stereohedron, MicroLine, MicroRidge..  is explained earlier in this post.

Conical stylus can't touch (damage) the area of the groove that Shibata can read easily. Even if the vinyl is slightly worn by previous owner during the 1970's it is most likely worn by conventional conical tip or an elliptical tip. Shibata can read the undamaged part of the groove on the same vinyl. This is the reason why i always buyin a cartridges with the most advanced diamond profiles. Another reason why i like the MM/MI is replaceable stylus, so i can compare used stylus to a new stylus by swapping them in 5 seconds (which is impossible with MC carts). For the same reason i like to try different styli on my MM carts to compare one cantilever/stylus combo to another. 

We're not talking about accidental damage, sh*t happens, i know. 
I believe an experienced audiophiles know how to set up a cartridge and a tonearm too. 

Simple FACT: LineContact type of the stylus has much longed life than conventional conical/elliptical, that's it. 

    

What is clearly stated in the manual of ZYX Premium 4D cartridge is that a LIFE TIME of the 3 um x 60 um (Micro Ridge 0,07 mm) stylus is 2000 hrs at 2g tracking force. I believe that Nakatsuka-San is qualified to say the truth even if he’s got a profit on every exchange of the worn cartridges to a new ones!

So 2000 hrs at 2g tracking force is a life time of brand new Micro Ridge stylus according to ZYX.

Buy cartridges with Micro Ridge or similar ultra high-end needles to use them for 2000 hours without retiping them.

I don’t know any audiophile who will stick to one cartridge forever, normally people sell cartridges before the stylus is worn out, but when we’re buyin used cartridges it’s nice to buy those with long life to insure ourself, it’s also nice to buy from a collectors who has many carts in rotation (then most of them have low hrs of use).

I can’t imagine myself using one cartridge for 2000 hrs, so i don't care. 
But i don't want to buy a used cartridge designed only for 300-500 hrs, especially the vintage MC which can not be retipped by the original manufacturer. The opportunity to find NOS stylus for MM is more optimistic. 

You normally don’t throw away a perfectly good $5k or more cartridge.

This is a typical MC dilemma

... and $5k is low budget MC as we can see nowadays, why not a $15k cartridge just for 1000 hrs as Raul said ? LOL

Official trade-in (instead of re-tip) is just about 60-80% of the new cartridge, good business :)

This is what we call premium segment