I read a long time ago that based on observations after the world wars, a rule of thumb was that it takes an army three years to discard lessons learned and go soft and unrealistic again, and it takes about two decades till the very last experienced NCOs and officers leave the army.

Most training in the field lacks wartime experience after a few years already, since even the most junior NCOs and officers with wartime experience move into cushier jobs than training green troops outdoors.