Go to Measurement->Settings... to display the dialog box shown in the figure below.
In , type the estimated maximum approximate reverberation time, if you click on the list, you get some alternatives.
It is also possible to type in another value that those.
Click the button to display help for more explanation.
In , note that the measurement quality usually improves with increased measurement duration. But an increased duration may be more demanding for the PC (especially if single sweep is selected as will be discussed later).
Setting sampling frequency and sweep range
Alternative 1: Measure using a single sine sweep
Alternative 2: Measure using multiple sweeps