Hi TonyG,
at first you have to separate specification limits of the instrument, defined for different time scales, versus sample acquisition time, following "Good Metrological Practice" (GMP).
Concerning the 5440B specification, you already have several different time scales, like
30days, 90 days, and 1year uncertainty, additionally the 10minutes, 24h and 30 days stability specification.
Both show different aspects and instability sources of the instrument, and these are defined on fixed temperature ranges, +/-5°C or +/- 1°C from the nominal lab temperature.
It's metrological practice, that the lab temperature is usually 23°C, but it may be defined differently in the calibration report.
The sources of instability are: Temperature, Ageing, Noise, and EMF, which you may mitigate, or simply include into your uncertainty calculations. The basic uncertainty of the Volt transfer from NIST to your lab has of course to be added; that's also mentioned in the specification.
The noise of the instrument plus your comparison instruments (e.g. a 3458A, or an external 10V reference) is the only parameter, which can be influenced or mitigated by different sampling time scales.
If you measure the 10V output of the 5440B for 0.1sec time frames, you will see a lot of short term noise. At 1sec ...60sec averaging time, these noise figures will decrease, and the readings will stabilize more and more to consistent values.
At longer time scales like > 5minutes, temperature and ageing drifts will already show up and will increase the uncertainty / standard deviation.
This whole picture can be summarized within the Allan Deviation statistics, which describes the stability on these different time scales. Maybe you check this idea with the vast thread "stability comparison of DVM", which TiN initiated.. there you will find a lot of such Allan deviation diagrams.
To measure the basic uncertainty, it is Good Metrological Practice to average over about 2..4 minutes, and also to determine the standard deviation (2 sigma) over all the samples taken within this time frame.
It's evident, that the lab temperature should not change during this measurement, at least not more than the given limits of the instruments used.
Frank