my Monos are eight inches long, I got them the second week in January '08 at 1.5 inches. People say 1 to 2 inches per month, mine have done 2 inches per month easy.
They grow very fast when young, especially given good filtration, agressive water changes, and the 87* F they like. I had one of my first three die and by the time my LFS got them back in, they were too small to go in my growout tank with my current monos.
In two to three months they will need to be moved out of your grow out tank (and may have eaten some of their tank mates)
Post pics of the newguy!!!