In an immersion context I can get to conversational fluency in about 3 months and to complete mastery in a year. I've done it twice, once for Spanish and once for English. A few things in my approach helped me move quite a lot faster than my peers:
1. I would carry a mini-dictionary with me EVERYWHERE. Anytime there's a new word, I would ask a local to teach me how to pronounce it and then make sentences with it while I was walking around. CONSTANTLY.
2. accent and good basics help more than a vast vocabulary: when I went to spain for the first time I would hear in the metro the famous male and female voices saying "proxima parada... something something" and I would repeat that sentence trying to imitate the pronunciation and rythm to get used to "sounding spanish". That helped a lot.
3. date a local: in spain I was dating this girl that was a journalist and from a pretty conservative family. She was very afraid that I would put off her family by being a foreigner and not being able to pronounce things correctly or making grammatical mistakes so she would correct me on the spot EVERY TIME I said something wrong. I dind't mind it and it worked like a charm. Years later I met my American wife that wasn't nearly as concerned about my pronunciation in English so my accent is not nearly as good as in Spanish, but I definitely learned the language, went from being barely understandable to business meeting in about 4 months.
3. Watch tons of movies with the original subtitles (for example spanish movie with spanish subtitles) to understand how people pronounce certain words. DO NOT limit yourself to learner materials, you won't learn a thing. Find something you enjoy and just dive in, you'll learn a lot quicker that way.
Dedication and systematic work is all you need to move pretty quickly, the human brain is wired for language, if you feed it what it needs it will do the work for you.