I think Doc had it right a lot of years ago :
when asked about the longevity of a White breech-plugs here is his answer :

How long should my breechplug last before I have to replace it?

Your White breech-plug should last virtually forever as long as you don’t lose it. They have been tested to 10,000 shots without damage to the nipple. The secret is that the nipple-breechplug is hardened to RC 38-40 while the hammer is much softer annealed 303 stainless. The means that the hammer may dent, which causes no problem, while the nipple stays in shape, never bending, swelling or peening.