Many factors involved... Keep up w/the WCs and quality foods will speed things up a little. Mine showed a little color @ the LFS @ 1.5-2". Once I got them home and in a permenant tank, they started to color up more in about a month.
As for coloring, mine I have had some color up at ~1inch (lwanda), some 5 inches (we will call it a sunshine peacock). It all depends on the fish and if it is assertive or not. (Which ties in with genes, food, fellow fish, water quality, everything.) And sadly you can have a fish fully colored up and the next day suddenly not showing much at all.