Little Nightmares features four chapters, each with its own theme and enemies. Most of this time is spent solving puzzles and exploring, although some intense encounters are also featured.
When all is said and done, you can expect roughly 3 to 4 hours of playtime from the game. This is assuming you play the game at an average pace, and perhaps die a few times.
Note that there are a set of collectibles which may bring this playtime up higher. Though, there is no multiplayer or new game+.