Outlast 2 uses a system of storytelling that encourages thorough exploration of dangerous territories. Because of this, one playthrough may take longer than another based on how many story documents you uncovered while playing.
The developers claim that Outlast 2 will take around 10 hours to complete, and my playthrough took around 9 hours with moderate exploration.
However, the achievement list for Outlast 2 reveals that the game can be completed in under four hours, granting you the Ashael achievement. This is at least double the length of the original Outlast which could be completed in 2-5 hours.