How long will it take me to learn Japanese?

I'm starting independent studies of Japanese and I want to know about how long it will take me to get to a decent level of reading, writing, and speaking the language(prefereably daily conversational level). I already know how to count, I know a few kanji, I know almost all Hiragana and Katakana, I know the days of the week, the months in the year, colors, some nouns and verbs, and some basic phrases. I'd say that I can learn pretty fast, because if I use a word or phrase in real life two or three times it's set in my mind. And if this makes a difference, I'm fifteen and my only language is English(but I know a few basic words in French, Spanish, Italian, Vietnamese, and Mandarin Chinese), and all I'm doing to self-teach is watching Japanese movies, listening to Japanese music, using flash cards, and reading a coursebook on Japanese as well as listening to Immersion CDs. So again, about how long would it take me to get a good level of Japanese? Sorry that this was so long. And thank you in advance. :)