This question is a little bit odd, no offense. Might with less than six months? Who knows? I cannot assure to you how long it will takes, I think that nobody can tell you with preciseness how long it can take. Nowadays, with less than two years, I can "dance in 30 seconds" and watch some videos without subtitles, some I can watch by myself, with no doubt, and other don't. Maybe because they are natives? You have to train your ears - listening, listening, listening and listening ... - in order to achieve that stage. I'm not a expert in this field, but I hope that these tips can help you. If you have any doubt, tell me.

There's not a correct answer to that question because it depends on a lot of factors. It depends on the person, the length and content of the video, the person's native language, etc. That being said, I have a lot of friends who have learned English just from watching movies and videos. It's really quite impressive, to be honest. American or British-made movies are great for learning English because it helps you put together the concepts you have learned in your head.