Several synchronous and asynchronous MAC protocols have been proposed for wireless sensor networks (WSNs). Among these, RMAC  exploits a control frame named Pioneer (PION) to set up the path and transmit a data packet through multiple hops in a single duty cycle. In this letter, we numerically analyze the average packet latency of RMAC in a probabilistic manner, which can be easily applied for any multi hop transmission methods. To evaluate the accuracy of the numerical analysis, we compare the analysis results with the ns-2 simulation results. Then we mathematically obtain an optimal duty cycle value that minimizes the average power consumption while satisfying a given delay bound.