To improve Wi-Fi performance in scenarios with high collision probability, recent amendments to the Wi- Fi standard introduce a number of channel access methods based on preliminary channel time reservations. A common feature of these methods is that channel time is reserved in a periodic manner. Unfortunately, having defined a number of channel reservation methods, the amendments say nothing on how much channel time shall be reserved to transmit a stream with required quality of service and minimal channel resource consumption. In this paper, we develop a mathematical model of streaming via periodic reservations which helps to answer this question.