A question came up a short time ago about how much wear was "too much" on the brass worm gears in the Leroy-Somer gear boxes used on the SM mainsail furlers. At the time there were no good answers posted.
I have been servicing mine, and had the opportunity to compare my installed worm gears (which are relatively new) to much older ones that had been replaced by the previous owner and put in the emergency spare parts box. The criteria to look for was immediately obvious with new and old side by side.
The attached photo will show the difference. The new worm gear has flats on the top of the teeth (on the right in the photo). As the gear ages it loses material from the sides of the teeth, and the teeth get thiner, eventually wearing until they are sharply pointed on top (on the left in the photo).
A good criteria to use for replacement would be when the teeth first lose that "flat top". After that, wear will accelerate. As the teeth get shorter, the worm engages less area on each tooth of the worm gear. As the teeth start to get shorter, the risk of them failing to mesh properly with the worm becomes higher.
I am sure a Leroy-Somers factory technician would have a more sophisticated criteria, but this seems a reasonable answer for our application.
Back Creek, Annapolis, MD