The duty factor is the product of the pulse duration and the pulse repetition frequency (the lapse of time the transducer is actively transmitting sound).
Most transducers are acting as the receiver 99 % of the time with a duty factor of 1.

See also Dwell Time.

(ITA) The time average intensity is the average ultrasound intensity calculated over the time between pulses.
Time average intensity = Pulse average intensity x Duty factor
For ultrasound systems, the instantaneous intensity is averaged over one or more scan repetition periods for a specified scan mode.