All I can observe is when the Demon Lord and Hero come. They come when the borders of countries are stable, when tribes and cultures are stable, when the same groups go on fighting with each other. When few great discoveries are still being made in the academies, and those discoveries that are lauded are of little account in changing kingdoms. When laws and customs and artifacts have channeled the course of behavior into few permitted paths, and the rulers of the underworld have tightly controlled illegality. When the world's rough form freezes into place, then it is ended, even though living beings go on changing and never is the same soul twice brought into being.
Whatever answer would satisfy that unknown question, it has not been found in any of the forms that have ended. Every stable form I have witnessed seems to have proven equally unsatisfactory to the World or the mechanisms above it. There was never a civilization allowed to last for an eon before it fell. If the Hero wins too many times in sequence, the Demon Lords grow stronger.