Theoretical estimates of the lifetime of uni- and biporous microfiltration membranes as applied to the filtration of polydispersions have been obtained. The optimal microfiltration regime that makes it possible to increase the membrane lifetime has been found. The data were considered in terms of the probabilistic sieving model of the process with regard to surface pore blocking with time, which was recently developed by one of the authors. An explicit formula has been derived to determine the lifetime of a membrane with an arbitrary pore size distribution.