This kind of question is what old school historians who wanted to make history a hard science asked. The real answer is that there is no defining rate. Every war is different and is affected by factors too numerous to count. Some wars (e.g. WWI, the American Revolution, the Imjin Wars) are caused by systematic tension simmering for decades, while others (e.g. the Mongol Invasions of Persia and Europe, Spanish conquests of the Aztec and Inca empires) break out almost overnight between peoples who were completely unaware of each other's existence prior to conflict. But, in almost every war, there are grievances and tensions that reach back decades or even centuries, that when combined with proximal "sparks" can lead to war. Or not. Different things happen in different times amongst different peoples for different reasons. There are no dependent and independent variables that we can nail down, and then collect data upon them and graph them one against the other like an economist or political scientist might.