Its not that I have a bad memory or don't know my own capcacity, Infact people often comment on how good my memory is, especially my factual memory. I write a lot, as in handwriting and as in story writing, and have tried languages before.anyway I've always had a good memory.
but to come back to the first question, the questions about how long japanese can be learnt are often asked on forums and the only really answer that can be given is that it totally depends on your memory (or even on the kind of memory you have)
If you tend to have a memory that use your hearing (sorry I don't know the english word for it), learning kana and kanji will be harder for you.
I advise you to try defining what kind of memory you have (hearing, eyes, writing...), because I think that japanese is one of the language that needs the most memory.
I didn't try learning Kana all at once because I knew that it'd be a waste of that day because I'd have to keep coming back to it later. So I've taken it very slowly.
I was told by a friend last year to learn so greek (for some reason I did) and I followed the same procedure with the greek alaphabet and that took me a month.
(write out each character you have learnt on aline every day fit as many as neatly possibly on that line and so on)
Of course I wasn't really all that interested in greek, so my interest faded away. But whenever I read something greek now, I can pronounce most of it because I still remember the entire alphabet. That's why I was hoping it would also work with this.
But as I said, some say they did it within a week or a few days, which as I said is pretty cool. B)