Thanks Kirt for your answer, but does it really depend on the instrument ?
An explanation is given on http://ku-prism.org/video/JohnPaden/network_analyzer_oper.htmhttp://ku-prism.org/update/SummerSeminar/MrPaden.html
According to this, the number of points is determined by the maximum length of the impulse response and the bandwidth of measurement:
number of points = length of impulse response * bandwidth of measurement
First of all I don't understand this simple formula, secondly how do I know the impulse response of my DUT ? Generally, that's unknown I would say.
The necessary IF bandwidth is determined by the desired dynamic range (SNR).
Finally, the sweep rate is calculated by the bandwidth, number of points and group delay through the IF BW-filter (approximately the reciprocal value of the chosen IF BW). The sweep rate, together with the length of the impulse response results in the change in frequency during the impulse response, which must be lower than the IF bandwidth ("the sweep time is always okay if you are properly sampling the frequency domain!").