Transceiver characteristics, asymmetric interference, and various properties of electromagnetic propagation environment create unidirectional links in wireless sensor networks (WSNs), especially in extreme environments. Utilization of unidirectional links is shown to improve network connectivity and lifetime. In practical WSNs, link level data exchange is performed through handshaking. However, a special handshake mechanism, where acknowledgment (ACK) packets are conveyed through a multihop reverse path, is necessary to be able to utilize unidirectional links. In this mechanism, hop length of the reverse path is a key design parameter because by allowing longer reverse paths, more unidirectional links can be utilized. However, increasing the reverse path hop length increases the energy overhead due to the extra ACK packets. Furthermore, complexity of maintaining the data flow also increases as the allowed maximum reverse path hop length increases. In this paper, we seek the answer for the following question: what is the optimum number of hops allowed for the reverse path hop length in WSNs? We created a novel mixed integer programming (MIP) framework to characterize the impact of reverse path hop length on WSN lifetime and performed extensive numerical analysis. Our results show that reverse path hop length has a significant impact on WSN lifetime.