Since July of 2010, actually. I was bored one day, and decided to google "worst thing in the world". Lo and behold, SCP-100 shows up, on page 2. I was intrigued by the page, found other SCPs, and the rest is history.
I would have signed up earlier, but, honestly, I was frightened off by the standards of the site. I look forward to the creation (and inevitable) destruction of my first SCP.