it will take a very long time, if it is breeding. if not breeding or ever bred, should only take a year tops. breeding cons at a young age will stunt their growth.
When he hits about 4 inches he will slow down in growth. Mine has been breeding since he was smaller than the female he was paired with. He is about 2.5 years old now and is about 5 inches. He had hit 4 inches within the first year and has very slowly grown since then. Good Luck!