I was thinking more about this lately and it occured to me that 36 hours or a multiple of an entire day plus half would be the best length for an initial dark period. There was a study done on resetting the clock during the dark period (which I can't seem to find at the moment) where they had the most success resetting in a 12:12 light dark cycle at the 6 hour mark. with a 2 hour window. Take this with the fact that the rythm persists (although somewhat diminished) after a full day of darkness, then you have 36 hours as an excellent choice.