Mine seems to be growing sorta sputy. Had him for a few weeks and didn't notice much growth, but in the past abnout 2 and a half weeks he's grown about an inch. He's still pretty small though so we'll see how it goes.
A lot depends on what you feed, how often you feed, how big your tank is and how often you do water changes. I've also noticed that different species grow at different rates. Tems grow faster than monos.
For maximum growth feed well, feed often, provide as much room as possible and perform large, regular water changes. I change 70 gallons daily on my 125 if that's any indication of how often...
I have been feeding mine guppies. there only about 2 inches right now but i only had them a little over two weeks and they have grown about a half inch or better.