As distributed systems and computer networks become more sophisticated, the problem of accurately estimating the system traffic intensity increases in importance. Information about the traffic can be used, for example, by the network manager in load-dependent routing and system reconfiguration. This information can be gathered by carrying out window-based surveillance, i.e. traffic intensity is estimated by counting the packets transmitted over a window. The authors show how to compute the optimal window size if the traffic intensity parameter changes with time.