Neck sizing only has just about fallen by the wayside.

Most accuracy nuts (myself included) now are full length sizing every time with a .0015″-.002″ shoulder bump for minimum working of the brass, but yet enough to assure no problems chambering the loaded round.

Otherwise…when the loaded cartridge gets hard to close the bolt on, when chambering a round, time to full length size. Number of firings before that happens is anybody's guess. Many different variables will determine that.