Semi Off-topic, but answers the question:
I was born in '94 and as an only child, there were no relatives older than me to get me into SSB early. Though my first console was a SNES at age 3, my mom blew it up (accidentally) and so I was stuck not knowing the true greatness of Nintendo. At age 6 or so, I got a PSOne and liked the Crash Bandicoot games. However, I had gone to one of my mom's friend's house on a party and played SSB64 with the kids there. The memory is quite vague as I only remember playing as Pikachu in Yoshi's Island. Later on (I believe Early-Mid 2005), I FINALLY gotten a Gamecube, (My GBC and GBA had slightly-rekindled my liking for Nintendo). As I first game, I chose between SSBM, and Crash Tag Team Racing. My naivety made me pick the latter. A short while later, I bought SSBM and Mario Sunshine, fully rekindling my Nintendo Fan-side. I played SSBM, and with my Mario Fan side up, mained Mario for a good couple of years ('till 07 or early '08), though I had no skill in ATs, save for shorthopping and fast-falling which I figured out myself. During that period of time, I played Smash on and off, having no idea the smash community was this big (or this good) and I was more interested in the main Nintendo games. Still, I was better than all of my friends. That's when IT happened. Going on YouTube, I happened to come across the video "SSBM hardcore match". The Falco performance by Bombsoldier was an amazing spectacle in my eyes. That got me wanting to learn more of smash. So what did I do? I switched my main to Falco and tried to learn ATs. At first, I was discouraged. Despite viewing Wak's AHTP vid, all I got out of it at the time was dash-dancing (though it worked on some of my friends, I still felt rather bad at the game). And thus I had quit again. It was only recently (mid-late '08 that I got a better understanding of said techs.). Now knowing SHFFLing after failing miserably for a long time, Wavedashing and (just a couple of months ago) Wavelanding, I dominated my friends further, even in Brawl and 64 where I wasn't as proficient. Knowing these techniques, also gave me a sense of confidence in this game, and now I play it bi-daily at least, regardless of if it's vs CPU or not. Knowing these techniques, review Wak's and the Bombsoldier's video eventually led me to join the Smash Boards officially (I had joined once before but only to get the SSB Mugen), watch and study more combo-videos, enjoy the fighting genre much more, and with a couple of thread I had read a few of weeks ago, got me to remake Mario/Doc as my second (and now possibly first) main.
But if you want the answer to this question in a much simpler way: I'd say about a timespan total to one year. (24/7/365 timespan)