If it is only what you said, then it shouldn't take more than 1-2 months
Welcome to Japan
It is absolutely pointless you doing anything off your own back before joining because it won't reduce your time in Nippon by one minute. Even guys who already have a JCAB ATPL (ex JAL etc) still have to experience the 'joy' of a 6-8 month course, as they are 'trained' (read prepared for the farce that is the JCAB way) according to their new airline's way of doing things.
I can only speak for AJX. You'll spend the first couple of weeks in the classroom having lessons in air law and radio stuff. They are quite 'specific' in their teaching and you will be well prepared for the exams but it does actually require a fair bit of homework. Assuming nothing's changed since I did it, you'll sit an in-house radio exam (theory stuff, not practical) and an out-of-house air law exam. They'll also be mucking around with alien registration cards etc but that's not ATPL related.
Once you've done those exams it's ground school time. This is where the type-rated guys shave off about 3 weeks which, I'm delighted to say, applied to me. It's CBT with progress tests along the way. There is certainly not a great rush to conquer the systems knowledge and the ground school could be halved in time but you're banging your head against a brick wall if you think you'll achieve that. For those that are unfamiliar with the FMCs they have separate lessons that specifically deal with those of the 767. At the end of all that it's another exam, encompassing all you've learned (when awake).
The true theatre starts with the simulators. Here you do not learn to fly the 767, you do not learn to operate effectively as a crew and you do not do anything that a 'normal' airline would train you to do. No; here you learn to pass the JCAB checks. You'll only fly from and to one airfield. You'll never divert from it and you'll never land anywhere else. You will practice time and time again the 'set pieces' that the 1950s-era JCAB want to see performed. They can only test you on these things because most of them have never sat in an airliner as a pilot and as a Cessna pilot they have only learnt the 'standard' emergencies of the theatre of which you'll be a part. You will become an extremely accurate pilot when it comes to speeds and altitudes (which is good) and you'll do things all the time (circling approaches) which you've probably only seen a couple of times in your life before. Your hand flying will be good and your visual circuits will be a thing of beauty. On the other hand you will only fly from the left seat (right seater is always PM), whether you're an FO or captain and when in the left seat you will be expected to be the 1950s captain that everyone in the 'normal' world does not want to see any more but the JCAB cannot change to accommodate that sort of modern thinking. You will 'order' your PM to do things (and be castigated should you utter such pleasantries as 'please'), you will make the decisions and will announce them as your 'intentions' without reference to your mate in the other seat and you will handle everything and definitely not manage the workload by delegating some of it to the PM. Japanese FOs are trained as radio operators and occasionally they are allowed to touch the controls. They are not trained as future captains, as per a good deal of the rest of the World. This means Japanese FOs are not capable of making decisions (please note the sarcasm in the way I write and don't take it literally). Of course, since it applies to Japanese FOs, it must apply to all FOs, right? So the JCAB expect the Captain to do everything apart from the radio.
Having bashed the hell out of the simulator and managed to squeeze a 2 week course into 2 months, you'll finally be ready to take your type-rating check. Then it's more simulating as you prepare for your ATPL check. Remember, I was type-rated and it still took over one month for me to 'do' 14 simulator sessions. These were the full-flight sims, not the fixed base ones that come beforehand
For the 'short course' it only took me 3 months from joining to ATPL check; expect to add another 4 weeks or so for non type-rated.
After all that it's medical time and, normally, a period of about 10 days when you can go home. Sometimes the timing changes and sometimes you can get longer but this is Plan A for most courses.
Having recouped and learnt not to speak Japanese at the supermarket check-out it's back to ground school; this time to 'learn' all about the airfields you'll fly into. A bigger waste of time and effort I'm yet to see. Hour upon hour of slides and monotonous droning about taxyway this-that-and-the-other at Poofrigginbingbong airport. It's an excuse for the Japanese to do their colouring-in and highlight everything on their Jepps....you'll also be expected to do likewise. It doesn't matter what you actually highlight; just scribble shite all over the place and they'll think you're studying hard and nod their heads approvingly....more theatre
After all that you'll actually get to jump in a real aircraft and do the route 'training'. You'll learn nothing from the Japanese pilots, only what you should have done. The only time you actually get told relevant 'stuff' is when you're flying with one of the 'Western' RTCs or when you'e talking to your mates in the hotel. Depending on which seat you're in the route training bit can take a few flights for FO (during which time you learn how to operate the radio) or longer for the left seat. It took 2 months of flying for me; not necessarily because I'm THAT bad but because Xmas got in the way for a bit and the JCAB couldn't get their act together to do my check.
All in all, it took me almost exactly 6 months from joining to doing my JCAB check. The non-type rated guys took nearly 8 months. There are a million ways they could shorten the course and they've been told it countless times. They've even been warned that it is a reason many pilots are not even interested in looking at jobs in Japan but it seems to fall on deaf ears. They are very 'traditional' i.e. that's the way it's always been done and if it worked for the YS-11 then it will work for the 767/777/787. Factor in the Ancient Ones at JCAB and you are not going to change anything, especially the length of your course, by doing things in your 'spare' time. Much better to just enjoy the ride; you're getting paid a full salary during training (unlike KAL etc), so just take advantage of the extra time in the training to do a bit of exploring, get the family out for a visit and have a few beers with the guys who are in the same boat.