took my 55 about 3-4 months to turn ar sand live. that's with over 120lbs live rock, so i kinda overkilled on that. but now it is working really well, there's tons of shiznit living in the sand. i've actually sprouted some little quarter inch clams that are everywhere in the sand now. worms, copepods, and other stuff i'll figure out later sometime.
if you can get a bit of live sand out of an established tank it'd go a lot quicker though. i'd do that, my lfs will sell a couple cups of ls cheap as heck.