Length of game is immaterial. There is no specification or standard required for a "milsim". Some will do well with a point at which it should end, others will end when the objectives have been completed, however long that will take.
Milsim is such an overused term as well. I agree with Leth that what I consider to be "milsim" are few and far between. There are certainly more seriously minded games out there, such as the OP No-Nods and Nightfall, but I would not count those as "milsim". They are still games with two equally sided forces fighting for essentially the same goals, with the same command structure. While these games were quite fun, the milsim to be had were in the private moments with small teams making their own experience milsim. Those teams that did that were able to continue through the entirety of the game, and I have no doubt would have continued to do so indefinitely until the conditions for stopping were met.
The key to making longer games last the whole time is to ensure strong leadership. Strong leadership at the heads of each faction, supported by strong leadership running the smaller units of folks beneath them doing the work, supported by strong leadership in the sub-units, and so on and so forth. Those that are able to keep track of their people to ensure that they are ready to endure the whole event will prevail, and they will maintain their entire force or close to it for the duration.
It's not the length of the game that matters, but it is the quality of those present, particularly those who are charged with leading others. A good leader will ensure that even if his least prepared member is adequately prepared or adequately looked after once it's apparent that this person's preparedness was lax.
Everyone is capable of doing the whole event, given the right rest, motivation and morale. This shit isn't hard, and it's generally, at most, 24 hours. Kid's play.
If you are not a leader, you still have a responsibility to ensure that you can endure the whole event. You can make preparations for yourself, and you can ensure that you are adequately equipped to take care of yourself, and your buddies. The problem is not the length of game, but in fact the mindset.