What is the average lifespan of a phono cartridge?

Bpoletti is exactly right. The stylus is what wears out. On MM cartridges, the stylus is 95% of the total price of a new cartridge, and in some cases it’s actually cheaper to just buy a whole new cartridge if you shop around. Stylus profile has a lot to do with stylus lifespan, because of the size of the contact area. A conical / spherical profile will only be good for 300 hours, while a elliptical could go for 500, and a line contact 800 or 1000. Depending on how frequently you play records, you might need to replace the stylus because the bushings become dry and brittle from age before the stylus tip wears out. 

Here’s a question... what does a worn out cartridge look like...and better still...sound like?
if we all agree that just one pass of a worn out stylus will ruin your record, then determining that your cartridge is worn out...is going to cost you a record..or more.
Interestingly, to me, there must be numerous folks who are destroying their precious vinyl, precisely because they are altering the groove with their worn out stylus...and will not even truly realize this...until they play the same LP with a new cartridge...and hear the damage that they done...which is now irreversible.

One side benefit in the design of Decca and London cartridges is they contain no rubber dampers to dry out, if such a thing actually happens in those that do.

When a stylus is worn, or the suspension has begun to fail, it will not track as cleanly as before.  This is first evident on more difficult to track passages, such as when a female vocalist its a not really hard or gets a bit too close to the microphone.  When you start to notice this sort of change, and cleaning the stylus does not cure the problem, it is time for a replacement.  It is NOT the case that slight, occasional mis-tracking from wear will ruin the record; records are not as delicate as some would make them out to be.  

There is no really good way to determine the extent of wear other than listening for it.  Visual inspection is nearly impossible to do right; often a visual check will not show wear or other problems that are quite evident from just listening.  

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.