According to the 25 motif blog, the challenge began in August, 2006. Since it is still running, that makes 8 years for the challenge - and there doesn't appear to be any sign of it stopping, or even slowing down!
There are 82 past participants, and 40 current participants... and if I can ever get time, I would like to join this - it looks like a great opportunity to challenge oneself, and see what others are doing!
Just click on the 25 Motif Challenge header at the top, and it'll take you right to Sharon's site!
Aside from the 25 Motif Challenge blog is owned by Sharon, she also has another tatting site of interest - which you can find here.
She offers tips, tutorials, patterns, and a large gallery - full of lovely photos of her work.