mine didn't start showing until about 6 months. I'm at 9 months now, and i still have very little coraline... i only have florescent lights, though. It all really depends on the individual setup. If you have good lighting, and a good level of calcium, it shouldn't take long for the coraline to start spreading everywhere. If your tank is lacking one of those elements, or if there are other hinderances, it would take longer.