Two years back when I started learning Selenium and I learned it in 2 months. I am sharing my experience:
It is a 40 hrs session, in less than this time nobody can teach it completely.
It will take you a minimum 2 months to completely learn to a professional level.
One of the best advantages of online selenium training is that you can learn it from anywhere. You just need internet.
I am mentioning the course link through which I myself learnt it online https://intellipaat.com/blog/tutorial/selenium-tutorial/, I found it very beneficial.
I got to know about this training course through the following youtube video tutorial.
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I Got their videos first and watched them for 1o days and then joined their online training by paying a nominal fee. After watching the videos, I had a background in Java and selenium. This strategy worked for me as I went to online training after having a background from videos. Their videos were over 100 hrs with lots of assignments.
I strongly recommend their training as it is as per industry standards and has helped me with my career.