Depends on the size of your pond.
For instance, under a normal circumstance, the water level of a 6’x8’x3′ pond may drop 1/4″ to 1/2″ inch every week because of evaporation. When there is a high wind situation, the speed of evaporation may be faster than you expect. Sometimes water can drop a couple of inches overnight. If this happens, do not panic. Just simply add more water to the ponds proper water level.