they're in the serras so it's not the same grow rate as the rb which are in the pygos. serras grow lot slower. i dunno about the exact manny grow rate, but if its the same as every other serra, its about an inch per YEAR till approx 12 inches, then it slows down.
im not a manny expert so it might be incorrect, but i'm pretty sure all serras grow lot slower than all pygos.
I have had mine for about 3 months. i got him when he was 4"... he is now 6.5"... so with my experiences with manny's the enviroment changes there growth habbits. larger less stressful tanks make them grow quick. i have him in his own 75 with lots of current and lots of little fish to chase.