I think that someone progressing through the story at a reasonable pace and investigating things as they come, then they'll have gone through a chunk of the Grove and probably a lot of the blighted village. That's a best case scenario since it's very easy to see a bunch of other stuff first as well. There's a decent chance they don't see it until we'll into the act, they're not exactly funneled there at all. So there's two other zones they might explore in the very best case scenario, not to mention they might go through the old temple earlier on. And there's a decent chance that they explore the general swamp area before going into the tea house itself, which is a decent sized area. I'd say generously speaking, including the nautiloid itself there's like, 8 hours worth of content that an average player could easily see before getting to Ethel, which is in my opinion far too late to be introducing a principle of the game's pacing like danger zone rests. Those should be introduced and reinforced as soon as possible, probably before the druid Grove if it were up to me.
I mean, if they want to introduce it sooner - and maybe they do that, since they easily could change that from EA - it would be in the crypt, where you meet Withers, Ethels is the bigger dungeon, where it has more impact, but they could easily do it with the crypt too and maybe with the spider matriarch cave system.
I think that the crypt is the perfect place to introduce such a system. The important thing with a system like this is that you introduce it early so the player can internalize it. They need to see it as a feature to take into account, not an aberration to the way they've been doing things for hours already.
