Based on the goal of maximum permeate production rate, a simple and practical method is proposed to allow operators to determine when to stop a semi-batch ultrafiltration system and initiate the clean-in-place procedure. The method does not need previous modeling or knowledge of the way in which the permeate flux decreases with time. Simple but adequate instrumentation is required for on-line determination of the optimal length of time that the system has to operate. Although the analysis is made assuming characteristics very frequently found in the clarification of fruit juices, the application of the method could be extended to other types of operations where progressive and significant fouling obligates to frequent nonproductive cleaning periods to keep the economy of the production level.