I've been told that learning languages is a great way to prevent Alzheimer's disease when you get older. It makes you seem more of an attractive person and increases your job prospects and ability to make friends. And, after watching some anime and other weird stuff on youtube, I've gotten this urge to learn Japanese all of the sudden and just be immersed with its culture. I don't think I'll do it for VCE, I'm a bit late for that but maybe some time in the future, just for fun. So, here's my question, how long does it take to learn fluent Japanese for an adult?
What about other languages? Is the amount of time taken roughly the same?
As someone trying to become fluent in Hebrew through VCE, and raised in a monolingual house, I can tell you that the degree to which you master a second language is entirely dependent on two things:
1) the amount of work that you put in. If you're willing to write 3-5 essays a week, rewrite essay (really important for developing proper grammar and syntax), read articles online, listen to music in that language, and find people to converse with (in my case, random Israeli backpackers on Carlisle), then your language skills should improve in due course.
2) the teacher you have. Don't underestimate this point. A passionate teacher can really instill a love for the language, the culture and your extended topic. A teacher that you dislike, or doesn't teach well can be a major turn off (true for any subject). 
Ultimately, VCE success in LOTE is a mixture of many variables. Of course, if your passionate about mastering your said language, then you'll become fluent eventually, whether during, or after VCE.