In my experience they grow uber slow. Unless you are doing mad Discus like water changes they will grow pretty slow. It will be years before you see a juvie reach 10 inches....Well, if you had a ginormous tank that could help too...
I decided today that I want one as soon as my new tank is cycled. Guy at the fish store, who owns the place, said max size is six inches, but wished they grew bigger.
There are three distinct species of gold nuggets. One tops out at 6" but the other two do not. Definitely research your purchase BEFORE you get it home. I would also recommend waiting a month or two after your tank is cycled to let things stabilize. These guys are very unforgiving of burps in water chemistry. They also tend to do much better in warmer water than the standard 78 degrees that everyone tries them at.