takes a while.. Thats why dats at large sizes are expensive. I would say they an inch per 2-3 months. best way is to feed them a regular diet of feeder fish . Just q the fish . That and the clean water of course.
feed a varied diet, such as beef liver, krill, ghost shrimp, beef heart, blood worms, black worms. they learn to take these readily, and it is much healthier than straight up feeders... and less chance of disease. they are very slow