It takes about 18 months to get about 5 inches..with some getting bigger then this..and some being smaller.. usually the alpha male will grow bigger faster... in most cases.. but not sure of the growth in a 55ga... after 18 months the growth is even slower..
With good food and regular water changes you can expect to get a frontosa to ~5-6" in 12 months. Dominant fish grow quicker and sub dominant slower so to minimize this difference you really need a larger tank where they can escape each other if need be.
It takes about 18 months to get about 5 inches..with some getting bigger then this..and some being smaller.. usually the alpha male will grow bigger faster... in most cases.. but not sure of the growth in a 55ga... after 18 months the growth is even slower..