[stcredzero]

You know that scene from Animal House where the 60's college kids are sitting around smoking a joint, and someone proposes the idea that every electron is a universe, and their minds are all blown? That's a parody, but it also really is what a human mental model refactor looks and feels like. (Both from the outside and the inside. Yes, I'm speaking from personal experience.)

how can you know when you have completed a refactor?

Before you've completed it, other people might think you're annoying because you want to talk about one particular thing a whole lot, and they're waiting for you to get over it. Most likely, you'll roll back some of the changes and make some bug fixes, and you'll stop feeling "whoa" and euphoric, and people will start treating you normally again.

The above answers:

why would a partial refactor be bad for the mind?

Probably good for your mind, but an interruption for your social progress.