They are listed on various sites as anywhere from 6" to 12" but usually with qualifiers, i.e. "Maximum size is 10 inches, but rarely exceeds 6 inches" seems to be a common statement. My book says "rarely over 15 cm (6 in.) in the aquarium, 25 cm (9-3/4 in.) in the wild."
I have never had one grow more than 6 inches.
I have never had one grow more than 6 inches.