Measuring the duration of a wave (or half wave) does not strike me as a good way to measure the frequency of a signal, because it's very susceptible to minute changes in the timing of detecting the start and end conditions.
A far better way IMO is to measure the time taken to receive a number of pulses (and make the number high enough that the error due to timing resolution acceptably small) or count the number of pulses received in a period (and make the length of the period long enough that the error due to timing resolution and reception of incomplete waves is acceptably small).