How long will it take to learn and become a Blockchain Developer?

You need to know and learn about blockchain very well, and you also should learn. As an entry-level blockchain developer, you should be ready to put in a lot of effort.
If you are a complete beginner, including the code, You need to get familiar with the coding, mostly with javascript. This may take two to three months to qualify for an entry-level job. Then it would be best if you learned blockchain software and concepts like Etherium, Etherium Smart Contracts, Meta Mask, Node, Hyperloop, Open Either, etc., and familiarity with basic blockchain contracts is necessary. This may take you around 4 to 5 months doing courses. In the end, it will take around 6 to 8 months to be ready completely. You can also do your job and learn it.
But for, an experienced developer will understand the code and its corresponding jargon and can grasp the terminology faster.