[KyleGoetz]

It depends on your age and how much you devote to isolating yourself from your native language.

You can live in Japan and speak almost exclusively English (with a little "supermarket" Japanese) and not starve. But you won't have a lot of fun.

If you stay away from your English-speaking classmates and only speak Japanese, you'll learn it fast. I know someone from Thailand who went to Japanese as a high-school student speaking it very little, and he was in my fifth-semester Japanese course at a university in Tokyo within 6 months.

Join clubs, watch TV, maybe date a Japanese person, study hard, go outside, and you'll learn Japanese quickly. You won't be a native, but you'll speak it well. Especially if you're under 20 years old.