In the wild it can happen over about 2 years. The same can be said for the same fish kept in closed systems.... with optimal conditions. Maintain high water quality(Zero ammonia, nitrite, LOW levels of nitrate, pH/dkh, sg, low phosphates, etc, good water movement, regular water changes, etc), feed several(4-5) small feedings a day(dont over feed though) of a varied diet(raw krill, meaty fish, scallops, squid, clams, table shrimp, muscles, a good flake or pellet food or a vitamin/nutient supplement). Frozen foods lose nutritional value in the freezing process so mix them up rather than feeding him a steady diet of just 1 or 2 things. Avoid feeding him anything from fresh water, ie; minnows or goldfish. These will eventually do damage. Keep him happy and healthy, give him plenty of space to move and grow, and you should see noticible growth every 3-6 months. You should also see more distinction in coloring and markings. He should get to his 6 inches in a year and a half to 2 years.