I learned Japanese for a few years in college, with up to 4 hours of class a week (using the Minna no Nihongo books). I managed to get to an acceptable level (able to have daily conversations) and even had a short internship in Japan. Basic understanding is pretty easy with standard methods like books or even apps. But like others said, the hardest thing is to go to the next level, when you have to learn a lot of kanjis and the pretty numerous and complex variations in almost everything, in order to really understand real life content. I never went to this level, and then I stopped learning at the end of college. Japanese IMO isn't like English, you will lose what you know very fast if you don't use it regularly. It's a lot easier to use English than Japanese, so I lost a lot of what I knew back then. I understand more easily how Japanese people talk because of animes, games and movies, but I don't remember the kanjis and the vocabulary (so I understand the point of the sentences but not the subject ^^).
Anyway, I'm supposed to go to Tokyo in a couple of weeks (waiting for my visa documents right now) in order to study Japanese and work there for 2 years, because it is ultimately the only way to really learn the language. 20 hours of school a week and work with Japanese coworkers are surely needed in order to speak and understand almost fluently, especially because of kanjis that you will remember only by using/seeing it all the time. I even remember a Japanese engineer during my internship who admitted that he was really bad with kanjis, it can be hard for Japanese people too.
