We've been running 17hr games out here that have worked out really well.
Having planned and participated in games ranging from 1 hour to 36 hours in my 13 years playing this format hits the right spots.
Game starts @ 10PM and goes until 5PM next day. Allows long term players to indulge in the night time portion then we have daytime players come join us at 10AM the next day.
We've found that players tend to play the night portion until around 4AM ish giving a good solid 5-6 hours of play then a minor downtime until 8am.
For the most part not many people tend to drop dead from this format as it has a natural rest time and when daytime players come to reinforce allows the full time players to take a small break/eat/etc.
The biggest factor to any long format game is ensuring that commanders are on the ball and properly rotating/resting folks as best as they can. It's sometimes tough to tell your average airsofter "Hey take an hours break" when all they want to do is go-go-go. and think that they are going to miss out on something. Those type of players are usually the ones who gas out the earliest so game organizers sort of need to take a bit of that into account.