In recent studies, the lifetime of Wireless Sensor Network (WSN) has been paid more attention to in order to evaluate the network performance. In this paper, service-oriented lifetime is proposed to provide a new method of assessment. The deployment of network can be guided by this assessment through placing more nodes where higher service quality is needed. In this way, the wasted energy can be reduced efficiently. To give a concrete scenario validating the feasibility of this assessment, a quantization process of network density is put forward subsequently. Both the theories and the simulations show that this method is feasible.