Appearances aside, it doesn't seem like he's changed that much.

[He still acts like Muscovy, and even talks the same way Muscovy did. At least he still remembers who they are, because Rey doesn't trust herself to try and rebuild whatever it is they had.]

When I went back home, it was for a few weeks, even though I don't think I lost any time here at all. Heard others have had similar experiences. Just... not to that extent.

Guess that means there's no limit to how much time someone can be "gone".