There have been wildly varying accounts on the length of The Order: 1886, but a full playthrough has finally been posted by YouTube user PlayMeThrough, and it's pretty interesting, because it's basically a "bare minimum" walkthrough. He doesn't stop to examine any of the many story-related items that can be found around the game, and almost never dies. 

The full playtime accounts to about five hours and a half, which definitely puts to rest the "rumors" about the game being three hours long. And that's without enjoying a lot of the content included in Ready at Dawn's title.

On the other hand, cutscenes cannot be skipped, as they have a lot of interactive elements within. We also learn that you can change difficulty dynamically at any time while you play, without needing to restart a chapter.

Interestingly,  PlayMeThrough himself mentions that the game feels "much longer than it really is," which is often the case with heavily story-driven games.

Of course, that's not the time a normal user will take to complete the game, as many would probably need to try quite a few challenging parts a multiple times to achieve this kind of "perfect run," and most will stop and catch the story clues, which take a reasonable amount of time (and some are absolutely hilarious, especially one in Nikola Tesla's lab, but I won't spoil it).

In fact other users that have played the game report over social media and forums times varying between eight and even as long as fifteen hours (for example, one of the first players to achieve the platinum trophy took about nine hours to complete his first playthrough, even if he had to go back to find some collectibles afterwards), if you spend a lot of time getting immersed in the beautiful world, look at everything there's to look at, and if you die a reasonable amount of times.

That said, no more room for rumors. If you run through the game with full sails, without being sidetracked by story and flavor elements (again, I definitely don't advise it, but to each his own), and you're much better at shooters than I am, this is how fast you can make it.