regal angels are not particularly fast growers. it also depends on how big your tank is. most people who report fast fish growth tend to be the ones with larger tanks where fish feel more natural. i'd say a good, long-term environment for him would be a 72"x24"x24", or 180 gallon. In a tank like that, I'd expect 1-2" of growth per year until it hits 6-7". Then the growth really slows down but the fish will slowly grow to full adult size of 8-10" over the next 5-10 years or so.
In a small tank, fish's growth usually gets stunted and you may see no growth at all. it's common to see a fish with a 10" adult size to never get past 5" in a smaller-than-recommended tank, even after years and years of waiting. eventually they suffer premature death.
of course, diet is important too. regal angels are spongivores. feed it a wide variety of food, including ones containing sponge (such as the ocean nutrition's angel formula).