About 2-3 years or so of getting tips from more talented people. (and a year before that when I started)
Started out when I was around 10, borrowed my bro's laptop and messed around with an old version of FL Studio in my warm, sweaty room. (the kind where it's not pleasant to have a burning hot laptop around)
I think some of you guys who've been WIP forum regulars can remember some of my first remixes...
(Jeez louise that's real hell on anyone's speakers.)
My first learning experience was when my brother taught me how to do drum beats in FL Studio because I wanted him to teach me music. This ended up becoming a jazz-ish kind of song using some of my drumbeats and a flute chord solo I wrote (he taught me how to do chords, but I never remembered them and relearned them last year in 8th grade) I never really did anything until I got my own computer around 2-3 years ago, and I sucked because I barely remembered anything he taught me, all I knew was the piano roll, channels, and step sequencing. It just evolved from a horrible techno-y Megaman 7 intro stage cover into a Megaman Zero 3 remix where I got a bit rude on my WIP forum (and was heavily flamed for it). I got depressed and deleted all of my stuff, and then a week or so later I came back. I posted up a Serenade of Water/Requiem of Spirit tribal/techno remix on the WIP forum. My bro then gave me a very interesting lesson on rhythm, and I made a Metroid Prime Remix. I learned more about how to make good arrangement structure, and then I just made more remixes, each one a new lesson on how to make OCR material music. Not quite there yet, but I talk to some talented OCR guys now who give me more in depth production tips and such.
But it doesn't take THAT many years, it's more of a dedication and ability to apply what you learn and how quickly you learn it.