Depending on average life span, fish will typically get half their adult size within the first three years. Fish growth then slows down and may take several years to obtain full adult size. From what I've observed for a couple of decades with animals in general, not just fish, the longer the life span the slower the growth rate under optimum care conditions. Bad water quality and poor nutrition can stunt growth.
So if a fish has an average life span of say 5 years, it's going to grow up fast. If a fish has an average life span of 20 years or more, it will take much longer for it to reach full adult size. Bala sharks have an average life span of 10 years and they can reach an average length of 12". They can reach 6" within the first few years, but will take them the rest of their lives to reach 12". I remember someone trading in a full grown bala shark. It was the biggest I've ever seen in any aquarium and was 9 years old.
So growth rate depends on life span, water quality, and nutrition. With such variables, it's hard to accurately measure.