If you know the basics of React, it will take you between one and six months to become proficient in React Js. This depends on your prior software development experience and the time you can invest in learning. If you dedicate 10 hours a week to learning React, you should be able to master the fundamentals within a few months.
I recommend this React Js Training Course by Intellipaat to learn React Js.
Also, watch this video on React Js: