i have been trying these more and more. they are pretty hard to get right, and take up some time. i still don't think i get them right each time, its a different learning experience each time.
but usually i shoot between 800-1600 iso. aperature depends on what lens im using, but usually 9-11, sometimes all the way closed depending on light situation.
i think clouds are pretty cool with stars, but only if they pass by, and don't linger. the best star timelapses are on a clear night, IMO.
personally, i think 2 hours is a good amount of time for the stars. my shutter speed is 30 seconds, so running 2 hours gives you around 8-10 seconds of video, depending on framerate. i think 2 hours is a minimum actually. i love it when you see the rotation completely.
hope that helps. here are a few star timelapses in this video --