One concern that a lot of researchers seem to have in coming to Japan is the issue of functioning in a country with not just a foreign language, but also a different system of writing. Will there be trouble navigating the city, and how about things like getting an apartment or phone service or even understanding your mail? At ELSI, we're fortunate to have a lot of help and support, so even if these things were beyond my own ability when coming here, the things that needed to get done were still doable.
Behind the pragmatic issues though, there's still the concern of what day to day life might be like, and for many people if they're going to live in a country for an extended period of time it's really important to get to a point where they're comfortable in the language, so as not to feel isolated or excluded. Japanese has a reputation for being one of the hardest languages to learn, but at least in my experience it seems that most of that reputation has to do with an artificially high standard, focusing on attaining mastery rather than functional considerations of being able to have conversations and things like that.
Part of the reason for the reputation of difficulty comes down to learning kanji. Even with people born and raised in Japan, it's not uncommon to hear 'yeah, we find kanji difficult too'. At the same time, when you read about Japanese study, foreigners learning kanji seem to have turned it into a bit of a masochistic rite of passage -- it's very common to see people say things like 'oh of course you have to learn how to write them, the proper stroke order, each of the readings, etc' and 'well, you need about 2000 before you can do anything' and even stuff like 'oh, just because you remember a list of stuff about that kanji doesn't mean you know what it really means!', not to mention the convoluted discussions of what systems and methods you should use to learn them, how to retain them, etc. I won't say that learning kanji is easy. But I do think it's misleading in terms of what you need in order to function, compared to what you need in order to claim to have mastered Japanese.
The other element that is usually considered to be difficult is the grammar. There are some surprises there for speakers of romance languages (the big one being the heavy use of particles and the chain-conjugation of verbs to create fairly complex compound verbs). But I've found in speaking and listening that unless you deliberately try to be ambiguous or terse, peoples' understanding is pretty robust to making mistakes, and people aren't going to immediately jump to using causative-passive tense and the like when talking to you. There is also often a big worry about being accidentally rude, since Japanese has multiple different politeness levels, but generally it's the polite form that you learn and use first in classes, and the other forms are there for when you want to experiment with being a bit more casual. While there are more formal, super-polite forms, there are very few situations where it would even be appropriate for to use them, much less expected (that said, you'll hear it all the time in stores and train stations).
When it comes to the pronunciation and hearing of the language, it's actually pretty easy. The sounds of spoken Japanese are very regular and are quite similar in terms of phonemes and pronunciation to Spanish - enough that when I was learning, I constantly had problems confusing words from the two languages. There's a small amount of tonality and cadence, but less than in most other languages and not usually indicating sharp differences in meaning. There are exceptions, but they're usually going to be really obvious from context (e.g. koshou can either mean 'pepper' or 'breakdown'). Vowel length is maybe a bit more important to meaning in Japanese than in other languages - if you have a habit of extending vowels, especially at the end of a word, that can create some confusion - but if you roll your r's a bit or make errors of that sort, it doesn't seem to get in the way of understanding that much.
My experience at least was that spoken Japanese was very quick to become accustomed to - picking out word boundaries, and then the individual sounds. Granted, I'm a bit of an otaku, and I had seen a lot of Japanese media beforehand, but I had never actually studied the language before coming to Japan. People can speak very rapidly, and that is pretty difficult (but that would be true in any foreign language), but people don't have to speak quickly and usually will slow down in order to have a conversation with you if you're having trouble.
Most of what I had to adapt to was getting over the uncertainty of actually trying to speak, for fear of making a mistake or saying something rude. That period of delay in searching for words 'how do I say the thing I want to say?', 'what is the right way to compose this thought?', 'what is that grammatical form again, how do I say this correctly?', etc is the biggest hurdle, because there will be dozens of conversations you could start or join in on, but that delay will often make you default to English or just stay silent. So the main thing to learn is, just make mistakes, make as many mistakes as you need to in order to get yourself to try to speak. Once you get into the habit of not stopping yourself, it becomes much easier to reinforce what you know and to begin to stretch it. And it gets you to a point of being able to function quite quickly.
In terms of time, I've been in Japan about a year. My ten years or so of watching anime was probably worth something like a month or three of classes in terms of where I was when I first arrived - I was comfortable enough hearing things that lessons sounded familiar rather than new and alien, and I had picked up some random vocabulary or had some memories of 'oh, that sounds familiar' to draw on. I wasn't at all comfortable speaking because all of my previous exposure to Japanese had been passive, but after another three months of class I had gotten a lot of practice with the feeling of not knowing how to say exactly what I wanted, but saying the closest thing instead anyhow. At that point I'd say I was pretty much able to function in day to day situations - not knowing every word or nuance, but enough to actually hold a conversation with someone, ask directions or details and understand the response, or explain a situation.
When it came to reading, I had to make a conscious decision to pursue it. For day to day things, it would be very easy to make do with just speaking but effectively be illiterate - at worst I might be stuck asking the staff at restaurants to pick things for me. What I found was as follows. There are three writing systems: hiragana, katakana, and kanji. Hiragana is used for Japanese words and verb conjugations applied to kanji - its very squiggly and looks complex, but this actually makes it easier to remember. Katakana is used for words imported from other languages and is very angular and blocky, but many of the characters look very similar to each other and I found it harder to pick up than hiragana. Learning them, as in memorizing the characters and sounds, takes a week or two. Out of the two, katakana is probably the more immediately useful, since often you might be able to read a hiragana word but not know it, whereas katakana words are almost always English cognates. The main thing with both hiragana and katakana is that even when you learn them, you'll be very slow at first, which makes tasks like scanning for a particular item on a menu very frustrating. A few months of regular use (for example, if your class hiragana and katakana exclusively in your study materials) and you'll likely get up to a reasonable pace. In this particular case, I did find writing them to be a bit useful too, as it made them more automatic for me, and therefore helped me get faster.
Kanji is a whole other story. There are all sorts of terrifying numbers associated with kanji: 50000 kanji total, 5000 kanji for a university level of education, 2000 kanji to read a newspaper, etc. These numbers sound like a lot, and honestly they are a lot. I mucked around a bit trying to spot-learn kanji the way I had picked up most of my Japanese vocabulary (e.g. when I encountered a word I didn't know, just make a mental note of the experience and look some of them up later), but that was not getting me anywhere. Trying to go through written materials and look up each kanji one at a time wasn't helping either. So eventually, I decided to try to use a program called Anki to basically just cram kanji into my head. I don't mean blind memorization - I focused on decomposing the kanji into radicals, associating the radical combinations with stories, and then associating the kanji with at least one Japanese word I knew that used it if possible or otherwise using the English keyword for its meaning; I didn't sweat it if I didn't know the stroke order or all the readings or all the compound words using the kanji or things like that. The main point of the program was to schedule kanji for me to learn each day, according to a big ordered list (in this case, based on the ordering used in the Japanese school system, but there are other orderings that people go back and forth over) and based on my previous mistakes. The program kept the workload to something like 30-45 minutes a day and made it accessible to me on my phone.
The whole thing worked much better than I expected - I could just do the cards the program gave me each day while going to and from work, and I was able to pick up from 10 to 20 new kanji every day with a good degree of regularity and retention - currently hovering around 93%. So that's only 14 years to finish the lot, right? Turns out, even after a few hundred kanji, I was encountering them everywhere I went. After 500 kanji, I could get the gist of advertisements and things like that. And at 1000 kanji, which took about 3 months, my written vocab actually exceeded my spoken vocab and my grammar abilities - I was starting to encounter sentences where I actually knew every kanji but I couldn't figure out the meaning - so for now, I've stopped there and changed focus to expanding my actual spoken vocabulary and sentence comprehension. If I were to do anything differently about it, the main thing would have been to intersperse common or useful compound words (jukugo) using the kanji as separate cards in their own right.
So where does that leave me? Well, by the standards of a language study program, I'd still be considered a beginner at Japanese. Japanese is, after all, a hard language to master. But at the same time, even as a beginner, I feel like I have enough to not feel isolated. And that I have enough that now, I can learn more by interacting with people, reading things I would want to read anyhow, and so on, than I would have been able to before.