Up to the present, the operating time has been studied on only a single aircraft attacking a single target or multiple targets under enemy threats. This study is to determine optimal operating time and appropriate size of aircrafts attacking multiple targets. Measures of mission effectiveness is defined through derivation of the probability of the various events associated with operating. By using these measures, the expected benefit of operating and the expected cost of operating are generated as a function of time. To formulate operating time determination model, the expected gain of operating is defined as the difference between the expected benefit of operating and the expected cost of operating. The model can be used to determine optimal operating time which maximizes the expected gain of operating, and can be used as the basis for determining the appropriate size of aircrafts.