This excerpt was taken from my Quora forum. The original answers and more interesting content like it can be found at https://www.quora.com/How-long-did-it-take-you-to-be-fluent-in-Japanese-and-become-an-interpreter-Did-you-go-to-college-for-this-or-something-else
Fluency does not equal becoming an interpreter
Considering it is hard to define real fluency in a language, I presume I became somewhat fluent (daily conversational level) after 1 year of studying Japanese full-time while living here. I took classes Monday through Friday, for about 4 hours each day and, apart from using it at work, I refrained from speaking in English as much as I could (I was working at an Eikaiwa in the evening at the time). Since I was also living with my Japanese ex back then, using Nihongo at home also helped speed up the language acquisition process.
Within 2 years of Japan, I could naturally converse in a variety of topics in Japanese—ranging from politics to pop-culture, religion and video-games, human relationships and sports—without much trouble (if any at all). It was always difficult to handle topics in the field of the sciences back then; especially discussing things like systems engineering, robotics, physics and so on. What forced me to learn the language to discuss these topics was the extra translation duty I was handed by the university I started working at after my 4th year of Japan.
Once I started interpreting visiting professors and also taking teams from our university on conferences overseas, I was forced into an in-depth vocabulary work to have the terminology necessary to properly take care of my interpretation duties. In fact, most of the reading on language interpretation I have done indicates the need for further language study once you begin to interpret topics that are away from your field of expertise (Valerie T-Bouladon “Conference Interpreting” 3rd Edition 2011).
How does one become an interpreter in Japan?
Therefore, if you want to become an interpreter of the Japanese language, it is important to take into consideration not only being fluent in Japanese, you must also know your mother-tongue very well and you also need to be able to use the most current and proper vocabulary/terminology geared toward the topic(s) you are interpreting so that your interpretation sounds as natural as the source language it’s being interpreted from.
For that reason, I can honestly say that it took me at least 5 years in total to enable myself to become an interpreter of Japanese~English (and vice versa). In this first stage of my interpretation career, I could interpret topics related to general pedagogy and academia, business, sociology, tourism and systems engineering. It took me another 3 years on top of that to become a consecutive interpreter in the field of sports (and I am still learning it because sports is more fluid than other industries and constantly new terminology is being devised every tournament season). So you could say that it takes at least 8 years of living in Japan, 7 of these being fluent in the language, in order to be confident enough to call oneself a professional interpreter.
I could most likely handle simultaneous interpretation as well (sports interpretation is definitely more simultaneous than regular consecutive interpretation work), but that would take another few years of experience to develop the skills and confidence needed to do the job within the code of integrity professional interpreters must observe.
Please do not be discouraged by this answer if you are pursuing a career as an interpreter. It is a phenomenal field to work in with plenty of amazing perks, too many to count here in a Quora post. All I am trying to convey is that you must be serious about being an interpreter—just being fluent in both Japanese and English can’t make you a good interpreter. Yes, maybe fluency alone is enough to volunteer as a migrant translator at city hall or to do interpretation at the nearby international church (these are great ways to start acquiring experience interpreting BTW). However, if you want to be really serious about it, you need a strong combination of study, experience and a dedication to constant language improvement in order to really do well as an interpreter.
I recommend reading the book I mentioned above since it is a good overall introduction to the job of an interpreter, the different types of interpretation work conducted out there (simultaneous vs consecutive and their differences), as well as the skill set needed to be professional at it (book found here: https://www.amazon.com/Conference-Interpreting-Valerie-Taylor-Bouladon-ebook/dp/B006T69SFW/ref=mt_kindle?_encoding=UTF8&me=&qid=1537753422).
(Photos of me interpreting on several different occasions in my career)