In this paper, the lifetime of wireless sensor networks (WSNs) is modeled as a function of μ and , i.e. LT=f(μ,?), where ? is the mean value of energy dissipation when transmit one unit data to a base station or sink, and ? denotes the flow distribution in a 2D WSN area. The lifetime analysis for three famous routing protocols is presented in detail based on the model mentioned above. With the methods proposed in this paper, the average energy dissipation rate of anywhere and anytime in WSNs can be calculated. The metrics presented in this paper has been validated by simulation results.