According to the RFC, SampleRTT is measured once every RTT.
My text: Computer Networking: A top-down approach (6ed By: Kurose & Ross) also states, that:
Instead of measuring a SampleRTT for every transmitted segment, most TCP implementations take only one SampleRTT measurement at a time. That is, at any point in time, the SampleRTT is being estimated for only one of the transmitted but currently unacknowledged segments, leading to a new value of SampleRTT approximately once every RTT.
But when multiple unacknowledged segments are sent at the same time, and each may have a different RTT. How is it measured once every RTT, and not once per segment?