I am not really sure exactly myself, but if you give them a lot of room, keep the water good and feed them a lot, they grow pretty fast. All those factors are important though.
If you do that expect them to show mega growth you should notice mega growth up until 7-9in, then it gets much slower. Some shoot to like 9 then hit the stunt, some might do it at 7, just how it goes. All my pygo's went from 4in-like 7-8 pretty fast. Hard to remember exactly how fast, but 6 months or so if I remember correctly.
Impossible to determine exact rate though because you can have like 6 Reds 7in, then add a 4in, and a year later the 4in Red wont be the biggest fish, but actually might not be the smallest either. The 4in Red might hit its taper off size later than some of the others, and a 4in Red can actually pass a 7in in the same tank with exact same conditions. Might sound stupid, but I have seen it happen in my tank a couple times now, so it must be true.
So many factors account though, I mean I bought a loner Piraya from a guy that never ate. He had it over a year in a 90g by itself and he never got 1 in out of it, and he sold it to me for dirt because of that. I put the runt in with my big guys, and he became a pig. The fish that never ate never stopped in my tank loaded with bigger fish. The fish never put 1in in a year for him, and now has put on 3in for me, and tons of mass in 10 months.
Pecking order sh*t means a lot with them for sure. A fish that goes from top of the pecking order in 1 tank, to last in another tank might become a chow hound to change its place in the new tank.