Depends on what you want to learn and why you want to learn it really. If you want to learn it because you need it for work, then it's considered a survival necessity and you'll learn much faster. But if you're learning as a hobby, then it'll take a bit more time in theory. It can take anywhere between 1000 - 3,500 hours to learn Japanese. Learning to speak it is much easier than learning how to write it. If you just plan on speaking then you're probably going to spend anywhere between 800 - 1,500 hours, maybe.