It depends on the region, and even within a given region there are significant changes. For instance, the Roman Empire had huge standing armies, but after the fall of Rome, Western Europe generally lacked standing armies until the Late Medieval and Early Modern periods. China, on the other hand had standing armies continuously from classical times onward. This had significant effects on many things, such as how guns were used. People tend to think that Europeans developed an expertise with guns from an early period, leaving China behind, despite the fact that the Chinese invented guns. But recent research suggests that Chinese armies were much better at using guns on the battlefield than Europeans well into the 1400s. Why? Because you needed a standing army to train and drill gun soldiers to shoot in turns, and without shooting in turns, early guns were fairly useless. I think that in general you find standing armies wherever you have centralized states. Europe, with its feudal system, didn't see centralized states until fairly late.