How do tell when my stylus is too much worn?

Raul,
500 hours is certainly safe but may be a little extreme. I'm sure it depends on the manufacturer. I read somewhere that dynavectors are only supposed to be good for about 800 hours. I have an old karet with 3 times that on it & it still looks & sounds fine. Anyway, this is straight out of my current cartridge manual:
The normally lifetime of a Clearaudio HD Diamond is by careful treatment and correctly adjustment up to 8000 hours!
Notice the words "up to". However it also has a boron cantilever & I keep my records & cartridge clean so I'm hoping I'm set for a while. :-)  

Not to muddy the waters but doesn't Van den Hul suggest 3,000 hours for his cartridges?

Dear @don_c55 / @czarivey / @boxer12: The issue is " deeper " than manufacturer specs or what we think and experienced about.

Many years ago Ortofon published what they found out on the stylus tip wear. They make several tests and declared that the diamond tip starts to shows ( under microscophic. ) tiny signs of wear just after 250 hours when LP are well cleaned as the stylus tip. They did it ( if I remember ) with 3 different stylus tip shapes where the line contact " suffers " a little higher signs of wear.

In the past with vintage cartridges no manufacturer that I can remember spoke on 2K hours or more even not 1K hours and many of them spoke on those 500 hours and the stylus tip diamond ( as material ) was not different on the newer cartridge designs.

Now, we have think on the turtouse road and very high friction/forces generated between each LP groove and the really tiny tiny stylus tip surface that are in contact.
It’s true that diamond is at the top of the mohs scale: 10 for harness but even this is not " immune " to wear.

In the other side and at microscopic levels that’s where everything is happening during playback: a " tiny " scracht in the LP grooves it shows it for the stylud tip a if that " deformation " been the Everest mountain and needs to " figth " against it. The LP surface have macroscopic and microscopic scratches and other kind of " anomalies " that for the stylus tip is a serious obstacule where sometimes the stylus tip looks one of those anomalies as if was a big stone of 1 ton.

Over that LP surface is full of dust and it does not matters how well we cleaned the LP. Exist microscopic dust that we can't see it. As a fact it’s in the room enviroment and is attracted my static or just gravity inside the grooves. This kind of dust degrade the stylus tip life and certainly the grooves integrity.

Ok, we can’t hear or be aware of that kind of damage and what we listen is fine, we don’t listen any kind of " aberration " during the cartridge tracking job.
This is influenced by what @rrm/OP posted:

""" I suspect that the change would be so gradual that it might be hard to tell, as the ear slowly accomodates. """

and maybe because the room/audio system has not the level of resolution for we can be aware of.

If our target is to have and stay with the very high quality levels in our room/audio system then my advise is not wait over those 800 hours to re-tip the cartridge.
I think that 500-600 hours is what we have to take in count.

As I posted that is me but as always with any audio subject is up to you.

Regards and enjoy the MUSIC NOT DISORTIONS,
R.

Records are sometimes much more valuable than bloody needle, and if that's the priority one should disregard figures of thousands of hours and replace earlier. 500-600 hours is of course very conservative but why not? I would stay under 1000 hours, anyway.