How much Japanese immersion you will be able to get and how much time you can contribute?
If you were in an immersion environment only speaking and listening to Japanese people and put consistent effort in you could definitely go from zero to generally competent at conversation in one year.
Mastering things like written Japanese, formal business Japanese, poetic Japanese and anything else not spoken on a daily basis would probably take longer, depending on your effort. Also in practice, people tend to do things like burn out, slack off, lose confidence, withdraw into social groups of other English speakers, etc. It requires quite a bit of self-discipline.
So what’s immersion learning? It’s a language-learning method where you are surrounded by the language you are studying. Through immersion learning, you learn words naturally through context and repetition, and here you do not require to translate back and forth between your native language and Japanese. Immersion is accepted as one of the fastest ways to learn a language.
Along with this, what amount of time you are devoting to learn Japanese also matters greatly. Time and commitment are the fundamental factors of learning a new language. The more time you devote, the faster you will learn. That’s simple maths!