The best procedure is to take at least three consecutive dark frames so that you can median filter them to remove cosmic rays. You probably will want to leave these running in the morning when you go to bed; you should arrange in advance with Observer Support for the dome to be kept dark. BEWARE OF LIGHT LEAKS.
You only need dark frames if whatever dark count there is comes in the form of hot pixels or other structure. A smooth background due to dark count will automatically be removed when you subtract off the sky background. Making an extra correction for it using a dark frame only adds noise to the final reduced spectrum. So before subtracting the final dark frames from all of your hard-won data, it is worth checking that doing so helps rather than hurts.