This seems like a much harder problem than it actually is. The key is the undecided territory.
You know that you loose points when you play into your own territory (Japanese rules). You also loose points when you play into your opponents territory without requiring an answer. Consequently, virtually all moves go into undecided territory. Once the undecided territory is gone, both opponents will pass to avoid loosing points.
It is perfectly possible that beginners see some territory as still undecided when a pro would see it finished, and vice-versa. That's not a problem. As long as one player thinks there is undecided territory, they keep playing. For both it's just a part of getting better to realize "oh, I don't need to answer that move anymore, I can save that point", or "nah, if I play there, my opponent may not even answer it anymore, I can save that point".
My experience is, that the place where the undecided territory is gone is very easily recognizable after playing just a few games. And it will definitely change as players progress. The greater uncertainty for beginners is to agree on which groups are alive and dead. Especially, when seki is involved.
I guess, the question of when to finish is much more pressing when a strong player and a newbie play together. In such a setting, the newbie will frequently play end game moves that just loose them points. Simply because the newbie still sees some territory as undecided while the stronger player already knows to whom that territory belongs.
In such games, the newbie will learn to avoid playing moves that their opponent ignores, and thereby which territory is indeed still undecided, and which is not. In any case, it's not a big problem when players disagree about when to pass. It'll just turn into a learning experience for either player when they do.