The total duration setting is not used if Impulse/Frequency Response Advanced is selected as shown below, and you have selected Single sweep, Multiple sweeps or MLS as measurement type in no. 3 as shown below.
In this case you have to select the measurement settings manually by clicking the Sweep or MLS tabs. From this dialog you can set the sequence/cycle length and number of cycles/averages manually. We recommend not using the Impulse/Frequency Response Advanced mode since the user has very high flexibility and may set the parameters so an optimal measurement result will not be obtained.