There is a discussion going on at Neogaf -forums about the length of The Order: 1886. Now everybody's favourite source of inside information in the gaming industry, shinobi602, has made his comment on the matter:
"8-10 hours, 10-12 if you like to take your time, take in the sights and a explore a bit", he writes on the forums.
He also notes that playing at different difficulties will change things up a bit. This is not a surprise, as more difficult settings in action games will usually require you to take more time in combat encounters. The enemies could for example do more damage, so you would have to consider exercising more caution.