I've only been writing Legend of Zelda specific fanfiction for about a year. As for fan fiction in general, I'd have to say at least 10 years. I started with Pokemon stuff. You will have to search far and wide and probably enlist the aid of the Wayback Machine to find some of my earliest, frighteningly bad works - but some of my older, nastier junk can still be found on the fanfiction network. I've left some of my "better" (relatively speaking) Pokemon fandom stuff there because it struck a chord with many people long ago and said people sometimes contact me on AIM or email to wax nostalgic about how glad they are to have found this or that specific old fic.
I think where I really got good was in writing Trigun fanfiction. Been a fan of that anime/manga for about 7 years and have written a lot of short fiction for it. - It's very good for both deep philosophy and high action. A lot of the "style" I developed writing Trigun fanfics shows up in my Zelda fanfics (along with sometimes outright references!)
I've been writing original stuff... since I don't know when, but I've probably only gotten serious about it in the last several years. I'm more serious about taking constructive criticsm and polishing it up than I am about my fanfics - because I own the worlds and characters and because I'd love to become shiny published author someday. If you haven't seen my whining about it in the shoutbox lately, I'm getting durn sick of getting glib rejection letters from publishers and literary agents who obviously aren't even giving my proposals a second glance.
Anyway, I hope that's answered your question and thank you!