Well, if anyone is interested in knowing what it takes… I think in the Swedish school system we start truly learning English around the 10-12 age range somewhere (there is some English before then, but I personally felt like we didn’t have a lot of instruction in grade 1-3, maybe it started to pick up in 3rd grade, but anyway). When I was 14, I started reading more or less exclusively in English, and I was a voracious reader at least until my early 20s (after that I kinda fell off the wagon and now I’ll have a couple of sprints each year of reading a lot, like 10 novels in 2.5 weeks and then nothing/very little for a few months, then rinse repeat). At 16, I started playing WoW and joined a guild full of brits, so only communicated in English there. And when I got my (own) first computer (I had shared computers with my siblings) at 18 or so, I had the OS in English because I’d be more likely to find help online in English than Swedish. Actually my first computer was probably in Swedish and I got frustrated with having to translate help instructions online, so my second computer was probably the one in English. 
It wasn’t long before all my devices were in English, and since I lived my life mostly online, and the English language part of the internet is… ridiculously bigger than the Swedish, it was all English.
I never used SRS for English. I read a ton though. I once in my voracious reader days tried on the challange of reading 52 books in a year, and hit it in May/June, and that was my normal consumption. So for maybe about 5-6 years, I read probably 100 books each year, and I’d say that was probably the biggest contributor to my very extensive English vocabulary. I also read fairly widely, hitting both historical vocabulary through fantasy and some historical romance, and then a wide modern one through urban fantasy and romance (I didn’t start reading much crime until later; but I watched all the crime shows back then so I still had the vocab but through listening (and reading subtitles)).
So I guess, from my experience, by the point I can read Japanese books well enough (with some look ups) that I can start hitting 50+ books a year, well, then I’d be well on the way. But I’ll never be as fluent in Japanese as I’ve become in English, unless I suddenly have a very real reason to switch most of my life to run on Japanese like I did for English.
(And at no point in my getting fluent and reaching a huge vocabulary of English did I ever live in an English speaking country. Vacations, yes. And later I spent a few months here and there in different English speaking countries, but the only time I’ve stayed anywhere for longer than 3 months, have been Japan. xD)
Now that is probably not the most efficient path to that level of fluency. But I’d argue it is probably the more enjoyable since I never felt like I studied English (I mean in my own private efforts, school certainly made it feel like studying at times). I did feel it some for getting my speaking and pronunciation to a standard I could live with. And I struggled a lot with that during my guild days in WoW, asking my British guild mates to help me learn how to pronounce things and occasionally having them laugh at stupid blunders. To this day, I have problems with being sure I’ll say soap vs soup the right way around. 
SRS vs reading 500ish books. Yeah, SRS might be a lot more efficient.
And did I mention I’m not a fast reader? Oh no, I’m really quite slow… 
I haven’t thought about my fluency in this way before. Fun exertion down the memory lane.