My male and female convict have grown about 50% in the past month. But they are only about 2-3" long. Depends a lot on how often and what you feed them.
If you feed them a LOT they will grow relatively fast in the first year then slow down dramatically. However, if you have a pair that is breeding/spawning then it will really slow down their growth rate since they'll be spending so much time and attention on raising fry.
Here's some timeline pics of my male convict (yes, I wrote down all the dates when I took them, haha):