The discussion about The Order: 1886's length has been a hot topic of late. Estimates have fallen all over the place from three hours to fifteen, with one five hour and half run through turning up on YouTube yesterday.
Ready At Dawn have finally spoken up about the campaign's length and they claim it is closer to eight to ten hours for a normal run. Dualshockers report that at an event in Milan, the developer's CTO Andrea Pessino confirmed the run time, but also said that if you play on hard mode and look for everything you'll hit closer to twelve or more.
As I said yesterday, it shouldn't matter too much. If the game is good, the game is good regardless of run time. If you are worried about the value of the experience though, I'd hold back until a few reviews have come out.