How do you become a professional Flutter developer?

It’d take you approximately two weeks to get through it (or two days if you’re really passionate about Flutter). You’ll come out with a much better understanding of how Flutter works and the best practices for coding Flutter apps.

If you are highly focused and devoted to learning flutter, then, it would hardly take you more than a week . But If you learn it on daily basis, along with experimenting different things on the way of learning, then, it might take you around 1 month and believe me, this method is much better for learning.