The speed you learn the language is proportional to the amount of effort you put in and the amount of interest you hold. Learn the katakana/hiragana syllabaries as fast as you can. Go with a regimented program that has you learning a certain number of words per week, say 20-30(That's about 3-5 words a day, which seems reasonable.) Listen to the language as much as possible, in the form of anime, voiced eroge, etc. Start learning Kanji early. There are so many simple kanji you can learn, from the numbers 一 二 三 四 五 六 七 八 九 十, etc...to the symbols for 'day', 'year', 'month', 'tomorrow', and so forth. The best way to learn would be to just move to Japan and do immersion for a couple months, but if you're learning the way I described it should take a couple years. I don't think you can make it less than that without some hardcore several-hours-a-day shit. Immersion truly is wonderful...but I don't think it works for learning kanji.
I hope my rambling is of some use to you. I'm not fluent in Japanese, but I've been learning it for a long time, and am a pretty decent speaker at this point.