It took me about a week of fairly vigorous effort to start writing it fluently, but I also have the advantage of having seen and written many other programming languages. I have a few anecdotal examples of friends who are great developers, but still have trouble with Rust's ownership system.

When I say something like "incredibly", it's after thinking about trying to teach it someone more junior (like you'd see in a corporate environment with a mix of skill levels). I think that this would be a very difficult task.