EDIT: TLDR, because I realised I tried too hard to answer ‘how fast can I learn all grammar’ and ended up rambling a lot – I think you can read and study all structures up to N1 within a year if you have lots of free time. However, that’s not going to guarantee you’ll be able ot understand things, because the first hurdle to understanding Japanese is learning to parse sentences. Furthermore, Japanese has tons of idiomatic expressions and little structures with special meanings (e.g. だけに is something like a strong ‘because’. That’s not something you’d guess from how だけ and に usually are used), so if you call those things ‘grammar points’, which is what most people do… there’s really a lot to learn. Learning ‘all’ of it isn’t realistic. Learning enough to be able to understand lots of Japanese, and doing so quickly? That’s possible.
It really depends on what you call ‘all Japanese grammar’. If we’re talking about ‘grammar’ in the same way that it’s used in other languages, then… OK, basic modern Japanese verb conjugations, common functions of all the common particles, basic feel of sentence structure… maybe three months if you’re just reading and studying resources that explain them. There are people here who have completed Tae Kim’s Guide to Japanese, for instance. I don’t know how long it took them, but I think one year was probably enough time.
Here’s the problem though: Japanese grammar is, at least in my opinion, a lot less ‘centralised’ than European language grammar. Chinese presents the same sort of problem: zero conjugation, but lots of things to pay attention to in terms of sentence structure and common idiomatic structures that have special meanings. Those are things you’re not going to be able to cram into three months. There are just too many. For that matter, never mind three months: look at all the questions that people, including those who have finished Tae Kim’s Guide, ask on these forums regarding grammar, even as they’re studying N1 stuff in the meantime. I’m one of the people who tries to answer those questions, and I haven’t finished any grammar guides…
My point is this: reading about how a structure works isn’t always going to be enough for you to understand it in a sentence. Some things require experience that you can only get from reading examples and taking the time to break things down. Also, one of the biggest hurdles you’re going to face right from the outset is ‘how exactly do I parse this sentence?’ It’s not going to be obvious because Japanese word order is completely different from European language word order, and it’s going to take a while before you’re used to holding entire chunks of information in your head in a new order that you’re not familiar with. (However, I suspect, from your post, that you speak German or a similar language, which could help since certain German clauses have word order that’s similar to Japanese.)
Anyway, in conclusion: start whenever you want, but I think the earlier you start, the better. The basics probably won’t take too long, and I think one year (providing you’re fairly free) is more than enough to be able to handle long sentences and so on. Whether or not you’ll know all the possible idiomatic expressions… well, you probably won’t, but I’ll just say that you’ll probably know more of them if you immerse yourself in native material than if you just try to use a grammar guide, even if some books are very good and really do cover lots of structures, including obscure ones. No language can ever really be ‘completely’ learnt, and Japanese has a lot of little special combinations with unique meanings that aren’t necessarily the same as what you’d guess from their constituent parts, so wanting to learn ‘all’ the ‘grammar points’ is going to be a tall order. I think it’s much better (and perhaps more realistic) to aim for a certain level of understanding, and that’s going to come from exposure, not just grammar study. (Final bit of proof: I have a friend who studied ‘all’ of German grammar within… three months? Six months? He was very intelligent. He wasn’t very fluent, ultimately, even if his sentences were perfect. Grammar isn’t enough. You need practice, so starting as soon as possible is definitely a good idea, as long as you study grammar and examples, real sentences and short texts etc.)
