What is the average lifespan of a phono cartridge?

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. 

@chakster
that is exactly right. I have a friend who has destroyed numerous valuable records that he owns while playing old SPU’s and other London’s etc., Poor guy didn’t realize that the stylus was worn out, even though the suspension seemed to be ok.The elliptical and conical styluses were so worn that after just one play the damage was done. He only recently discovered what he had done by listening to a new Clearaudio Goldfinger ---and then hearing the damage! Shame that somebody had strongly recommended the older cartridges and he followed their advice---blindly.

Agree with @chakster .  Different stylus profiles have different lifespans.  Also, the construction quality of a stylus can impact lifespan (crystal structure alignment).  The various line contact styles / profiles listed by chaster have a larger contact surface area that spreads the load over the stylus.  This increases lifespan.  A conical cartridge profile has a very small contact area, actually just a point of contact.  These wear very quickly.  

BUT NOT TO FEAR!!!   

Just because some damage might be done to a record due to a worn conical stylus does NOT mean that your records are totally ruined and unplayable.  A new cartridge using a very small line contact stylus or a cart rebuilt by Soundsmith or others using a very small line contact stylus will be tracing the record groove in a different area, typically tracking MUCH DEEPER in the groove.  This area may not have been damaged.  May not have ever even been touched by a stylus.

I recently had Soundsmith rebuild a cartridge using one of their very small line contact stylus / boron cantilever combinations.  When playing some of my collection that was damaged in the early 80s by a worn Shure conical stylus came out clean sounding and like new.  No audible damage or distortion.  The stylus was obviously tracking a "virgin" part of the groove.