Hey, amongst others, the Alliance Française (http://www.alliancefr.org/etudiants-individuels/cours-de-francais-2017) is known to dispense good quality french lessons for foreigners. Another way, would be to reach the japanese community in Paris via the Japanese Ambassy in Paris (http://www.fr.emb-japan.go.jp), it may be helpfull to learn french directly through your language, with people knowing what could be the specific difficulties for japanese speaking people in the french learning process. Just did a quick search on their website and found this organisation: http://www.yuai.fr/ which is supposed to help japanese people settle in Paris.
Anyway, best way to learn a language is in my opinion, to practice it. So If you're considering movin' to France, well... Just give it a try someday even if your French is far from proficient. In the meantime, you still can try to find French movies, TV series, etc... On the Internet. I'm sure you'll be able to find japanese subtitles from some of them. In my opinion, It's very helpful to get used to the sonorities of a language to learn it.
As for your last question on how long it would take you to reach a decent profenciency, can't really reply to that. Firstly because it depends on people. Some are catching it quickly, some don't. There's not universal timetable. And secondly, I'm no French teacher, so I really can't imagine how hard it is to learn.
Hope you'll find some help and responses to your questions. And wish you all the best in your plan to go live abroad.