Most pool owners wait far too long before resurfacing, usually because the pool still holds water and still looks blue from the patio.
The signs worth acting on are underfoot rather than overhead. If the surface feels rough or chalky, if staining returns within days of scrubbing, or if you are steadily using more chemicals to hold the same balance, the finish has usually failed even though the shell has not.
Caught early, resurfacing is a straightforward job. Left long enough, water starts tracking behind the finish and the repair gets considerably larger.