I know for personal experience, that you cannot really expect something specific, due to the fact that there are many different factor which plays a role in growth.
When i got my first piranhas, i was unexperienced, and they did not growth fast, due to stress and overfeeding, which made them **** and puke it all out again. (Pellets). They went from about 2 inch to 4 inch in 6 months, no succes! I got rid of them, and started out fresh! And they went from about 2 inch to about 7 inch in 6 months. That is Pygo nattereri of course.