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It all depends on how fast you learn and retain all the information you study. For me, it took about half a year? 6 months or so, not sure. But even before I decided to continue my studying (I stopped for a few years), I already memorized the kanas so the only thing left was memorizing some kanji and studying up some grammar.
Well, what I did for memorizing kanji was flashcards. The Heisig method never really worked for me. What worked for me though is memorizing the kanji by stroke order and sometimes splitting it up into its radicals and knowing what each radicals mean. I also have a notebook just for remembering stroke order. One page for one kanji, and I would write it over and over again using the stroke order until I've filled up the whole page. It helps, because later when you encounter handwritten kanji, stroke order will help you recognize what that kanji is. Let me tell you, reading handwritten kanji is both fun and frustrating. It's fun when you finally found out what it is, frustrating when you still haven't. As for grammar, read up Tae Kim's grammar thingy. I never really got pass a certain chapter because I was already taking Japanese classes at my college plus I understood most of what I'm reading anyways (Everything else is easy grammar-wise when you've studied the basics). Though with Tae Kim's grammar, the way he explained things was really... iffy. It was hard to understand, so if you have those at some nearby community college or Japanese center near where you live, go for it. It'll help.
The only way I ever learned grammar was in class and reading manga (and light novels. Light novels are great if you wanna study grammar since it just slaps you in the face with it every goddamn sentence *coughhyoukaughcough*). A nice thing would be to befriend a translator or native speaker (or at least somebody who has more knowledge than you) and ask them questions about things you don't understand, that way, you can correct your translations and learn something. And if you want to measure what you've learned so far, you should try taking the JLPT. I was supposed to take the N4 last year but I was sick, so now, I'm going to try the N3 this year while my friend is going to try the N2 again.
Wish you luck on learning! It's pretty fun when you know another language cause then, you can mix them up when you talk to friends who also know that language. Plus you won't have to wait for scanlation releases.
I learnt Japanese over the course of 6 semesters in uni 2 years ago but I never really got into reading raw manga at that time. It boiled down to simply not having the patience to keep referring to my dictionary to look up words I've never seen before. A few months ago, I figured that I should do something to make sure I don't forget the Japanese I learnt. And that's when I started translating manga!