This page of the guide describes how long Darkest Dungeon 2 is: how many hours it takes to complete it, how the story is structured and how many chapters it contains.
Darkest Dungeon 2 is a long game. The story itself does not take much time, but a single expedition can last up to an hour. It mainly depends on how many times you'll have to fight - combat takes up most of the game's duration. Ignoring the fights, however, will cause more difficulties later in the game (fewer Mastery points, fewer Trinkets and Combat Items).
Each game requires you to reach the Mountain and deal with the Cultists and their boss. You need to complete 5 Expeditions to get there. One expedition may take about an hour . You can't pause it, but you can choose safer routes - the fights take the longest time. In such case, you should finish the first chapter within 5 hours . There are 5 chapters in the game. Please note that the difficulty level increases with each next one. You should also take into account that you probably won't be able to complete a whole chapter at your first attempt.
As of its release in Early Access on 10/26/2021, you can complete only one chapter in the game. You can still complete it again to unlock the available characters and try to complete the level differently.
Based on these calculations, it can be assumed that the minimum amount of time needed to complete the game is about 30 hours, which can go up to even 60-80 hours .