At best 1/2" per month. On average, depends on diet, stress, and amount of water changes done over the course of the week, so around 2-3 years of age, average size is 9-10" total length. There are some outliers out there that get more growth in the first 3 years.
Do not try the power feed route. People tend to forget to increase water changes when they do this. Also, the poo on GTs are much longer than a carnivorous cichlid like an oscar but still shorter than a herbivorous cichlid like a Pearsei, so a high protein diet over long term will actually slow down growth. Common ailments to a wrong diet is stress and clear transparent poo (intestinal mucus lining). Rotate in a more algae/spirulina based pellet like New Life Spectrum Algaemax with every other feeding of frozen food or other pellet.