With a good diet it should only take about 8-10 months if they are being feed properly i.e. a variety of QUALITY food and about 2-3 times daily, once they hit 5-6 inches they are ready.
That is SOOOOOO far from fact! GT's will be ready to mate after they reach 5 - 6 inches. My female drops a batch of eggs every other month like clockwork. She's currently carrying some eggs around in her mouth as we speak. They usually have 40 - 50 fry per batch. It's gotten to the point where I don't even try to raise the babies anymore. Eventually, they become food for the other fish in the tank. Water quality plays a BIG role in determining the frequency of a pairs mating habits. I change a third of my water every Sunday and haven't lost a fish in so long, I can't remember.