By the nature of the process, projection welding electrodes deliver the pressure and current through a large flat surface area as compare to spot welding.
This results in greatly reduced surface wear on the face of the projection welding electrodes. Their life is generally much longer and require minimal maintenance. The wear on the face of the projection welding electrode may have little if any affect upon the weld quality. It may only affect the cosmetic appearance on the part surface it contacts. In many cases this is not an issue.
As stated projection welding electrodes can run a long time without dressing.
If projection electrode wear does impact weld quality it is time to dress. If the part begins to stick to the electrode because of cracks or roughness, it is time to dress. Watch your weld quality but expect some other problem to crop up and influence your need to dress the projection weld electrode.
When dressing is desired for whatever reason be it part surface, sticking or convenience a light resurfacing will usually suffice. This can be on the machine or removed and done in a machine shop.
See another article in this blog:
CAN PROJECTION WELDING ELECTRODES BE DRESSED EASILY?
There is no canned number or welds at which this will occur. It should be more than spot welding. After a few production cycles you will learn what your normal process issues are and know what to expect and when. A routine will be easy set up and maintain for your process.
References: RWMA – Resistance Welding Manual 4th Edition