As this article shows, it is perfectly possible to learn a language to a decent level (B2) in a year by studying at home. This is especially true if the language you're learning is closely related to your native language (e.g., English and French). You just have to consistently put in the time (probably an hour a day, at a minimum) and use a good mix of resources: a good textbook, flashcards (e.g., Anki), Italki (personal tutoring over video chat), radio, and some easy reading (childrens' books are great).
If you can go to a country where the language is spoken and are able to spend all of your time working on the language, you can learn to B2 level in much less than a year. I would say you should be able to achieve B2 level in a relatively easy (because close to English) language like French in less than 6 months.
If you can go to a country where the language is spoken and are able to spend all of your time working on the language, you can learn to B2 level in much less than a year. I would say you should be able to achieve B2 level in a relatively easy (because close to English) language like French in less than 6 months.