Tank size, filtration, how many goldfish, water changes, food, and water parameters will all affect growth rate. I had one of mine go from almost 2" body length to about baseball size in about 4 months. I would up the filtration to include a Fluval if that is in the budget.
Do you have a substrate? They are prone to swim bladder disease and eating off the surface increases that risk dramatically. I would only feed New Life Spectrum's goldfish food, it is the best brand hands down. Their goldfish food is sinking which is exactly what you need. The problem is if you have gravel it can fall between the gravel. I had a bare bottom tank. If you have gravel you can just put in a pasta bowl for humans to eat from. It is short but wide. Use an undergravel filter riser tube to put the food right in the bowl, they will learn very quickly what that tube means.
I did about 90% water changes weekly. Since they are used to having a certain mineral content in the water I would suggest something like Goldfish Trace from Seachem to simulate this. The pH should be about 7.5, and if it isn't like this naturally I recommend API's Proper pH 7.5.
What type of goldfish exactly? How many? How big so far?