It really depends on how you read the manga. If your gonna read online scanalations, then just knowing the basics are not enough. If you read the takoban volumes, those volumes usually have fumigana (little hiragana next to the kanji) However just knowing how to read it isn't enough. you have...
Hmm... for speaking and listening, it took me a year, I only spent a little time per week though, and it was more of a hobby to learn.
For reading and writing, it took me a month to memorize the hiragana and kana. As for kanji, i didn't really need to learn much since i know chinese anyway...
I learnt it by simply watching anime. this process took me a year of watching subs before I finally started to be able to point out mistakes in the subs. I dunno whether this helps, but try go to Japan for a holiday :)
The hiragana and katagana is pure memorization unfortunately. However, I...