How large to they have to be before...?

I have 28 Kenyi fry in a 40 gallon breeder tank with 330 gph filtration. THey're currently about .25-.5 inches. How quickly should I expect them to grow if I'm feeding them approximately 4-5 times a day with a mixture of crushed cichlid pellets, algae wafers, blood worms, brine shrimp (live and frozen) and Hikari first bites as well as changing about 9 gallons of water of the water every 4 days. How large do they have to be before they're considered "selling size"? I've had them for about 2 weeks now and they've double in size easily from when they were striped to now. They were starting to look a bit cramped in the little net breeders so I put them in a 40 gallon tank. The main reason I ask this is because I don't want to stress the darlings by having the bioload in the grow out tank to be too high for two long. I want to trade them in for store credit at my LFS but they'll only take them "if they can sell them" and if "they have room".