I'd guess that less than 5% make it to the highest levels. Likely because they had some ludicrous motive for learning Japanese, like understanding anime/video games/dramas/music lyrics, or finding a Japanese girlfriend. I would say that you should ask yourself: "Why do I want to learn this language?" It's going to require one heck of a lot of effort to learn and gain any real fluency.
This. I've been studying Japanese at varying levels of intensity since I was eleven years old, and most of the classes I've seen had a fair contingent of people sitting in the back, listening to J-Pop and tracing pictures of their favorite anime characters. For all their enthusiasm for Japanese pop culture, they struggled with the material and constantly complained about how hard it was. Learning Japanese is a key to understanding Japan, not just its pop culture; make sure this is what you want.
That being said, I'm going to share some of my thoughts on learning Japanese that seem to go against the common wisdom of Japanese classes I've taken -
Don't try to learn how to speak first and put off writing until later. I'm going to go out on a limb here and say that, if you are trying to learn Japanese, you should not ever use Romaji. Kana are pretty simple and should only take a couple days to get familiar with. If it takes you longer, no problem, but learn them before you try learning how to say things. The longer you put off writing, the more you'll get used to thinking of the language without it and the harder it will be. I've seen a lot of people have particular issues with Japanese writing, and I think much of the reason for this is because they keep trying to avoid it. If you embrace it from the beginning, it will be easier for you. This also means learning Kanji. When you write, use as many as you can and pencil in the kana for the ones you don't know. Learn about the structure of Kanji and the radical system instead of trying to memorize them as monolithic units (the way many Japanese classes try to do it). Even if you are more interested in learning to speak Japanese, take some time to develop reading and writing as well. If you can read Japanese, you also get far more opportunities to immerse yourself in the language.
This won't hit you right away, but when it comes time to learn about verbs, don't learn the polite forms first. Every Japanese class I've been in has done it this way and it confuses the hell out of people. You will be confronted with a lot of learning material that tries to throw -masu
forms at you first, which makes all the other forms of verbs look completely random when you go to learn them later. Japanese verbs conjugate based a highly consistent pattern of endings that is generally not explained in its own right in classes. I had a chance to teach a class that was having trouble with verbs, and this was all it took to clear them up. This will probably look like going too far ahead and making a huge deal out of a small part of the language, but if you can't conjugate verbs, you're obviously going to have big trouble. Unless you're facing a situation where you need to carry on a basic conversation in public immediately, take the time to learn things in a logical way instead of starting with the selected bits that a textbook writer thought you might need first in the wild. You're going for the real deal, not preparing for a business trip.
My overall point is to seek out the patterns underlying everything and learn using them as a guide. Too many Japanese classes (and language classes in general) try to start by pinpointing a few specific things and hiding the context because it is "too advanced," with the end result being that everything is hard to understand because little to no context is provided for anything. Pick up a few different books, search the web, etc. instead of relying on one author's idea of how you should learn things. And whatever you do, steer away from materials that try to teach you in Romaji.
There are a lot of great web-based resources for learning Japanese. I've just started digging around this site, but Tofugu is very well-designed and so far, I think the author has a good grasp of language-learning. Here
is his "100 Best" list of Japanese learning resources that could help get you started.