I live in Montreal, Canada, and I’ve become relatively fluent in French over time. I studied French in school just like everyone else in Canada, but school is kind of rubbish for really learning a language I think. I got laughed at a bit for certain phrases I learned in school that were either really old fashioned e.g. “Zut, alors!” or just not what people use in everyday life in Quebec (can’t think of an example right now, but there are several).
Anyway, not to say that school isn’t useful at all, because it gave me a good baseline knowledge of French. Just that once I had that knowledge, consuming real media such as local news and local shows and movies as well as conversing with local people about everyday things helped me way more to really understand the language.
I’m not really a prime example of achievement or anything, I mean I am pretty lazy about practicing because my francophone husband speaks perfect English. But yeah, everything people say about immersion is 100% true, it just is really tiring and frustrating (at least for me).
I think unless you really plan on writing serious literature or papers for school, it might not be helpful to do studying for studying sake after a certain level. I feel like you probably already know this since you’re a pretty high level (at least in Japanese).
I imagine for English it is much easier to do this because there is much more media available, but what I do right now is watch the news on TV in French with my husband. It is cool because there are always captions at the bottom to summarize what the story is about if I don’t catch everything they talk about. If there is a word I don’t know, I’ll ask my husband what it means and sometimes an example of how it’s used. When I was working in a French speaking environment, everyone I worked with understood English but spoke only French. So sometimes I wouldn’t understand, but I’d just ask them what a certain expression meant. I started compiling a notebook on expressions and words which were mostly colloquialisms that I would have never learned in school. It really helped me so much with my comprehension.
I can’t really tell you how many hours per day it takes, although for example, if you are working in a non-native speaking environment, you can expect to have around 8h per day of immersion. I would say that would be extremely helpful. If not, even just listening to media in the target language for a few hours a day helps a lot. My dad married a Mexican woman and neither of them spoke each other’s language at first. My dad managed to pick up Spanish by watching Mexican telenovelas, and just carrying around his Spanish-English dictionary all day (according to him). I think the key really is to practice every day in whatever way is available to you, even if it’s just finding videos on the internet that are in that language.
I hope that helps a bit, sorry for the really long post!