Well if you prospective is that of a race organizer Dave, you have to ask yourself who you are trying to attract. A one to 3 hour can entice sprint racers to try something different. A guy can iron man a race that length or can do it with one buddy and they can use a pick up crew without a lot of prep.
With the proliferation of Miati that are easy on parts it really does not take much work to run a short endure.
There is a much smaller pool of hard core guys who like the long races but if you have the right track and the right date they will travel to participate.
We run a 3 hour at NHMS. We went to 4 hour and then to 6 but it hurt entries. NH is not a track well suited to enduros. Tough on brakes and not a good rhythm so we could not seem to make a longer race work.