If you have nothing else to do but French and you want to get to B2 in 4 months I would literally do nothing but French from when you get up to before you go to bed. I don't know if duolingo would help you very much in this situation - to be honest, I don't find the returns all that great, compared to straight memrise/anki flashcard type learning and other forms of practice. I always recommend these books but for reading/writing progress quickly, I recommend Hugo French in 3 Months (have a look at amazon reviews etc). If you're studying all day, you can do it in 2 or 3 weeks. When I started studying French in July, I started with that book. Now I can - with work and some use of a dictionary - understand most news articles, and that's with significantly less practice than you will be able to get. It explains the majority of the grammar you would need to read articles in French, though it doesn't have a lot of vocabulary. If not that book, any book that aims to present grammar in a workable format for reading will do. Personally, I got a more in-depth grammar book to work on after finishing that book. I would start with that and/or a tape course for listening and speaking skills. Listen to the radio all day, read the news in French - as far as possible, communicate only in French. Start a journal where you write down what you did all day in French. If you know any French speakers, ask them to talk with you in French, etc, etc. Move to a French-speaking area as soon as possible. To be honest though, I don't know if this would get you to B2 level... But it's worth a shot.