A lot depends on the size of the tank and what you feed them on, aswell as how often you make water changes.... Anywhere from 3 to 6 years, maybe longer although having said that my biggest is home bred specimen is 5 inches and not even a year old. The breeders are 4 inches long and approx. 15 - 20 months old.
My current babies are now 4 months old and between 2inches and 2.5inches and their colors still haven't fully developed. This is interesting because even though pet shops sell firemouths at about that size with well developed coloration it suggests to me that its an age thing not size thing that determines color progress in Firemouths. Maybe mine are advanced in size compared to those shop Firemouths.