Lifetime of a wireless sensor network is affected by key factors such as network architecture, network size, sensor node population model, data generation rate, initial battery budget, and communication protocols. We investigate accurate lifetime estimation of large sensor networks using discrete event simulation. Our experimental networks are large IEEE 802.15.4 compliant wireless sensor networks (WSNs). We take an iterative approach and obtain the ratio network lifetime over battery budget (LBR) by simulating only small budget networks. Using the converged LBR, we are able to accurately and efficiently estimate the lifetime of a network with full battery budget. Furthermore, this approach to lifetime estimation is both necessary and efficient when the network size is large.