May be 3000 hours........but usually you sell it or damage it before that!
The magic number I've often heard is 1,000 hours. But that's highly variable. If your records are clean, and you keep your stylus clean, it could be much more. Some people claim that stylus preservatives (like LAST) can help, too. On the other hand, one mistake in handling a new cartridge, could reduce the number to zero.
The best way to check the stylus condition is with a visual check. Some dealers have a microscope. I've heard that you can use strong magnifiers in home, although I can't remember what to look for.
Sonically, the primary sign are problems with the treble on new records. Also, tracking problems with the cartridge towards the end of the record would be a sign of serious wear.
Age can also have an impact--some have claimed that a cartridge is only good for a year or two--although I'm under the impression that newer cartridges age better than older designs.
The suspension drying was the thought on my mind when I talked about cartridges aging poorly. I've heard that older cartridges did have a real problem after a while (some even said a cartridge was only good for a year.) But, the materials used have been improved--or so they say. (Famous last words!)
If your table and arm are set up correctly and your vinyl is properly cleaned, 5,000 hours of life are not uncommon for good quality (Class B and above) cartridges. Forget the magnifing glass at home...not enough magnification. I use a high power stereo microscope availiable at work for inspection. I also agree with Adle...you will more than likely swap out before wear out.
My silly response: It all depends on the cartridge. I sold a Linn LP12 with a Monster Alpha Genesis 1000 MC to a friend of mine years ago and he is still using it, I know that cartridge very well and it is still hanging on. The thing has got to be 8 years old by now. I also owned a Linn K-9 that only lasted me around 1000 hrs, I put it in a shelf for around 6 months and tried using it again only to find the suspension got hard (gave it to another friend so that she -yes a FEMALE audiophile- could re-tip it and use it).
I forgot the name of the writer but he was one of the 2-3 most famous names in the stereophile business, who wrote, "I have never as yet [in my audiophile career]wore out a cartridge" I think that the 1000 hours that the manufacturers give, is an arbitrary figure to cover abuse, dirty records and other unsavory conditions.