This depends on a lot of things like water temperatures, water quality, and food quality. Of course, fish grow faster at higher temperatures, but they also need to be fed more. Ideally nitrate and ammonia should be close to 0; this can often be accomplished with more frequent water changes. If they are just eating "fish junk food" like flakes and getting very little nutrition, it will take them longer to grow. And genetics. Some fish are just predisposed to grow faster than others.
What are their current sizes?