How do tell when my stylus is too much worn?

Of course I should remove the cartridge and view under a microscope but un mounting and remounting is a perilous business that I would like to avoid.

Looks like you will void visual examining of stylus and only limit yourself to finding stylus problems only by ear which is somewhat inefficient.
Very often you may find degraded performance of inner tracks including sibilance. Also in diamond stylus less likely diamond tip is affected, but more likely stylus mount and cantilever which is visible under jewelers loupe 

Dear @rrm: 800 hours? looks not excessive. Now, manufacturer can tell us that stylus tip can goes over 1K hours or even 2K hours.

Now, the stylus tip degradation can be faster when LP's are not well cleaned and when the stylus tip is used not well cleaned or both. These accelerates the stylus tip damage.

My take with any stylus tip is to re-tip after or near 500 hours. This gives me ( someway. ) certainty that my LPs surface can't be damaged for it and that thank's that the stylus tip has no more hours its performance will be really near as when 200-300 hours with almost no demerit in the quality performance of what I'm listening.

For me 800 hours says it's time to re-tip but this is me.

Regards and enjoy the MUSIC NOT DISTORTIONS,
R.

Raul

All cartridge maunufacturers must love you!

500 hours of use then discard is total BS!

Beogram turntables with moving coil are still around since 80's and sounding good. Most of decks with lost styluses had been mishandled by owner. Others perform like champions. Unfortunately for some reasons cartridges aren't made anymore for them, or worth the fortune. I hoarded at least several Beograms with needles and checked their styluses and noticed little to no gunk and no loss of suspension integrity that is including tip and cantilever mounts. 
To be sure I carefully applied CRC spray on each and every one of the vintage cartridges. The playback is exceptionally clean for outer and inner tracks. 
Diamond tip can serve forever, but suspension and cantilever is what wears out. In case of beograms with lightest arms, the suspension is never too stressed. 

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. :-)