Knock Knock Moe'Suckra

Oh wow.
I’ve been a fan of Prince of Tennis since 2006—when I first saw it on Toonami. 
At first I wasn’t really into it. I found it kind of boring. I wasn’t really interested in sports anime or manga.
But then I decided “Why not look it up? Give it a try?”
and I started watching all of the episodes in Japanese online.
And started to really love it.
Then I found the musicals and started to adore them and look forward to them.
And then I started collecting the manga, to see what I was missing—like matches that weren’t in the anime.

Before I knew it, I had grown to really be in love with this series that I originally wasn’t really fond of.
I had watched all the musicals—subtitles or not—the movies, I have all of the manga (all 42 volumes) and I reread them often, I’ve watched all of the pairpuris, looked up character info—I never thought I’d be so in love with the series, but I really did grow to be as much.

So—to answer your question, about 6 years. 
And I don’t plan to stop. :)