Like most fish, the growth rate will be determined by how much you feed them, and how often you change the water. Increased feeding means increased water changes. In my experience clean water is at least as important as quality food.
Second that! Absolutely spot on. At least as important.....
I couldn't say how quickly mine has grown because I've changed the frequency of feeding and WC's a few times now due to changing schedules etc etc. However the biggest growth spurt I ever saw from my Rotkeil were in the weeks following his eating my entire army of Cardina japonica :lol: I've always done 50% weekly water changes but back then I was on 2x a week 50% so he really put on nearly an inch in TL that month.
What you'll find however is that it's very intermittent in terms of how they grow - a lot of growth goes into filling out - the fish may grow an inch one month and next to nothing in terms of TL the next, however the general bulk of the fish will increase quite substantially in that following month.
These various factors influence the accuracy of measurement so really it's not something that can be provided as a general rule.
Why do you ask? Are you considering entering the world of Sev keeping? 8)