[Jihoon]

Much of how quickly you learn a language depends on what languages you've had prior exposure to. So, depending on the language you choose to learn, YMMV.

Total immersion, at least in the beginning, is only useful when your language of choice has linguistic similarities to what you already know. Otherwise, to get to a level where immersion even makes sense comes only after hard studying. Without this base knowledge, everything you hear will sound like complete gibberish...not really an efficient way to learn.

As someone who is fluent in Korean, I'd presumably have an easier time picking up Japanese here in the US than a foreign exchange student in Japan who only knows English (given similar levels of dedication/work ethic). That's because Korean has many similarities to Japanese.