Mine grew fairly quickly iirc. He shot up to around 6" then slowly gained a few more over the past couple years on a diet of sinking carnivore pellets mostly.
If you can avoid pellets then do so for cats. When I used to have sun cats they grew quite well but I like bigger cats and gave them to my brother. I stuck with my redtails I think he still has one about 6-8inches now. Been about a yr or yr n half from a few inches to 6-8inches not bad. I'll see if I can get him to post photos. Never fed any of my cats til this day pellets. Cut shrimp or tilipia or just cut fish in general and on occasion mussel/clam meat. Cheap at asian supermarket. Get a mackeral chop it up into lil pieces it will last you months. One mackeral at asian market like 2-3 bucks. But what works for me might not work for you. Good luck with your fish it's a lunker cat when it gets chunky.
I have 3 in with my CA/SA, they are feeding on NLS as a staple like everyone else, I had one at first but after doing more reading I got 2 more. The bigger one is about 6.5" long being fed once a day on NLS, got him at 3" about a year ago and the other 2 are about 4.5" got them about 5 months later at the same size. These are great fish and love following each other around the tank 24/7, and they also seem to enjoy getting in all of my cichlids faces too lol. I cant wait to have some 10+ inch monsters!