The emerging IEEE 802.11ac standard introduces spatial division multiplexing access, in its downlink version, to the wireless local access world. Through this multi-user multiple input multiple output (MU-MIMO) technique the access point (AP) can simultaneously transmit multiple independent groups of streams to different stations. The consequent precoding requires precise channel knowledge and thus regular channel sounding and feedback. The interval to choose for this sounding procedure is an important factor. A compromise between precise channel knowledge at the AP and reduction of the associated overhead is needed. The first is a physical (PHY) layer parameter and the second is evaluated at the medium access control (MAC) layer. In this paper we present a PHY+MAC analysis of the impact of the channel sounding interval on MU-MIMO transmissions for different scenarios.