The article analyzes a classical result of the optimal foraging theory – E. Charnov’s Marginal Value Theorem (MVT), according to which the population leaves the patch containing a food resource at the time instant when the average rate of energy consumption is maximum. Observations show that the real time spent by the population in the patch often differs significantly from thetime predicted according to MVT. The proposed model takes into account the state of food resources in the patch, permitting to formulate a new approach to estimating the time when the population will leave the patch. In doing so, we use the concept of food attractiveness of the patch introduced previously by one of the authors. It is shown that the time of the population’s departure from the patch lags behind the departure time estimated by MVT, which is consistent with real-life observations.