What is the average lifespan of a phono cartridge?

I see ads for used cartridges stating that its barely broken in at 200 hours. How long will a stylus, cartridge etc last before something wears out and must be replaced/repaired? Thanks.
Tough question. a lot of variables.
In general cart makers usually say 1,000 hours of play time.
That can vary. If you play 'dirty' records with a lot of snap crackle and pop your stylus is going to wear faster.
If you play perfect clean Lps and use a stylus preservative like Stylast your stylus is going to last longer.
I would say 2,000 hours is a reasonable life for a stylus if you use Stylast faithfully.

And how does anyone "know" how many hours they really played?
It is total guessing on most folks part. Unless one uses some sort of device to keep track, it is just a wild guess every time. (usually below the real time used, more often than not, half the real time IMO)

I use a mechanical clicker to count each LP side played. Then I average the sides out to 20 minutes. So 3,000 clicks is 1,000 hours. that is the best i can do.

I have a 27 year old Dynavector Ruby 23 which i still use on a spare TT, The original owner used Stylast faithfully, and he gave me the cart. I would have to guess the cart has around a thousand hours.. but it is fine.

To really know if a stylus is worn you need to use a magnifying device and actually look at the stylus.
Used to be many good HiFi stores had a microscope just for looking at the stylus. Not anymore. Same thing most stores now cannot really mount a cartridge with any sort of accuracy.

Anyway, buying a used cart is 100% trusting the seller. If you believe the seller....
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.
There are a lot of variable involved. Cheaper cartridges use synthetic diamonds that don't last nearly as long as premium styli that are carefully cut and mounted so that the crystal structure is oriented properly to minimize wear.

How the user handles records and cartridges will also matter. If records and styli are kept clean, they last longer. According to one manufacturer (Benz or VdH, styli can last as long as 2,500 hours if the records are kept clean). I've used Lyra cartridges for close to 2,000 hours with no problems or sign of deterioration in tracking ability.

The rubber suspension parts of cartridges can go bad from aging and exposure to environmental agents (primarily ozone). So, even if unused, a cartridge could go bad. That is the theory, but, I've personally never encountered such problems--I've heard very old cartridges that still sound good after long storage.

My way of determining when a cartridge is beginning to go bad is to find the very few records that are challenging to track properly and listening for changes in the ability to handle the very few spots where distortion can be heard when everything is less than optimal.
Hah Lyras have whole different story on longetivity. 2nd to none indeed.
My findings are similar to others as posted. I had 2 Benz Ruby 2s which lasted for about 4-5 years. I am now using a Transfiguration Orpheus which is 3 years old and still very good. I'm not sure you can wear out a diamond stylus if your cart/arm are set up optimally. With commonly available anti-skate adjustability and digital scales enabling users to fine tune dynamic tracking force, I think in general cartridges are lasting longer than they did 20 years ago. I have been led to believe that the first thing to go is the suspension and not the stylus. Mistracking can often be heard on the inner grooves of poorly pressed or poorly cared for commercial pressings. I have found little or no distortion on the inner grooves of audiophile pressings even after the cartridge has been deemed old or worn out.
This is a great question. I began buying classic used cartridges ten years ago when a new cartridge I just bought disintegrated as I was installing it. It just fell apart, the body from the mount and the stylus assembly. I always wanted a Koetsu Pro IV, at the time they were unavailable new so I took a chance on a used one. It was perfect, so I bought another one six months later and it worked great. Since then I've bought (and sold to try others) over 20 second hand "classic" cartridges and all performed fine. Go figure. I suspect people who trade cartridges are just trying something new and thus don't put many hours on them.
I have bought in 1998 a second hand mk levinson mlc-1, it has been retiped twice by Van den Hull since that date and it works quite well. When you send it for retiping the company can have a look at the rubber suspension part and change it. I use it around 4 hours a week on a big plinthed lenco with a OL conqueror mk II and am quite satisfied.