The useful life of a cartridge case usually ends with an excessively expanded primer pocket or a split neck, and several factors can determine the number of firings it will withstand before calling it quits.
When a case reaches the final step in its forming process at the factory, work-hardening has made the brass quite hard over its entire length. The head of the case is left that way for strength, but its neck and shoulder are annealed to make them softer and more elastic.
If you bend a wire coat hanger back and forth repeatedly, it will eventually break at that point because it has lost its elasticity. That’s work-hardening. The same thing happens when a cartridge is fired and then reloaded: Its neck expands away from the bullet, and sizing it back to its original diameter during the reloading process causes it to become harder—just like the coat hanger.