In the visual dictionary, Ahch-To is supposed to be like Dagobah where time is iffy/slower. So I don't think there is actually a solidified passing of time for Rey/Luke on Ahch-To. In fact, I think in the movie Finn and Poe even mention having x hours left before they're toast - and Rey is on the island for at least a few day/night cycles from what we see.
But what we're talking about is it's unclear to the viewer of the movies. Because it's never discussed - or even mentioned - in the films, the "time is different" idea does seem a bit ret-conny - both in terms of Dagobah and Ach-to, ie to explain things after the fact.
And I believe the retcon idea is that due to "force-powerful" locations, on Dagobah time passes faster, so Luke could have spent 6 months training there (ie so his skills make more sense than if he spent like ten days or whatever,) but with Ach-to it's for some reason the reverse - a force powerful location where time slows down.
I dunno - if it was an actual thing, why not make that idea 100% clear in the movie? Otherwise it's another "gotta read a book to understand the movie" thing. It's also a "after the fact, let's say time is weird to explain why those things don't quite make sense in the movie."