Every fifteen minutes is plenty, if you are cold calling. Some days turkeys just dont talk much, and ive found that when i try and crank it up on those days it just sounds unnatural. My rules of thumb is that the quieter the turkeys are on a given day, the less i call. An exception is when i'm running and gunnin. I will make sharp aggressive calls, but only a series or two in each location before i move on. If i find a very appealing area to set up, but still havent got any responses, i'll spend some time here, maybe an hour, and i will tone the calling back while i'm "cold calling."Check yourself on a watch from time to time. Fifteen minutes is much longer than it seems when you are trying to reign in your calling. Funny how that works in the woods, yet my half an hour lunch break at work always seems to be over before it starts!
