I think the definition of "milsim" has been skewed over the years due to a cargo cult-ish approach to it by people who didn't fully understand it.
Milsim is not about how long an event lasts, the complexity of the scenario, the amount of props on the field, or anything of that nature; Milsim is about a specific mindset which strives for a realistic military simulation. For video games, think OFP/ARMA vs. COD/BF, rF/LFS vs Need For Speed, or Falcon4/MS FS vs other "sims" with simplified flight models.
First and foremost, the player must enter a full temporary suspension of disbelief and play like that BB is a bullet that will kill him/her; the shooter mentality of running forward and getting shot being okay because you'll just respawn in a few minutes just won't do.
Usually the supporting elements should also be as realistic as possible: realcaps (or at least no hicaps), realistic tasking/mission objectives, etc. First and foremost the point is NOT just to get kills, it's to accomplish a given task. Shooting is just one of the many tools at your disposal (and often is not the best course of action). So players who are just out to rack up trigger time need not apply.
Extra props, complex scenarios, matching kit, etc all enhance what should be an immersive experience.
But it would be wrong to think that these elements alone equal milsim: you could have all that and only have a very lavish skirmish. Or you could have simple skirmish-like "drills" which are extremely milsim.