Technically I guess it would be when they start rutting or just before they start rutting, so it's slightly different from state to state. Though maximum horn size will probably be achieved before that because of the difference in mass versus length. As the horns grow longer the top end of the horn becomes harder and drier. So the horns will gain length and lose mass from May to September. The net difference will be a longer horn that actually scores less B&C. However, this effect can be mitigated depending on the weather, etc. In general the net outcome will probably only be about a 1 inch less of B&C score, but it can vary greatly.