Ressurection post, I see. :P
Either way, I figured I'd weigh in; 18 hours is a nice balanced number; any less and it doesn't have the day-to-night-to-day feel of milsims, and any more and you risk putting players out of commission.
Rather, I think of it in a different light: To start, at Nightfalls 2 and 3, I powered through the full 18 hours, though I took times to take breaks and to pace myself. However, at Nightfall 3, my way of dealing with squad fatigue (of those who didn't bail... o.o) was to hold up in the two-story inside an unlit room with a flag in it, and to let the guys sleep. They set up a metal magazine at the door, designed to tip over at a slight movement. This arrangement let them sleep with confidence, as they didn't have to fret too much of people sneaking in to change the flag, or to attack them unawares. It also allowed them to contribute to the fight while doing absolutely nothing. It's strategy like this that makes 18 hour milsims work, but unfortunately most people's ideas are to sleep at the HQ/spawn in such games, and wait until morning. That usually leads to tears later on.