As you would expect with a game that draws a lot from Dark Souls, Mortal Shell is a game that will test both your skills and your patience.
As an action RPG, the game is designed with a lot of knowledge, which is often brought up throughout your interactions with NPCs such as the statues of Sester Genessa and the inscriptions you will find throughout the game. There are lots of places to explore and the dark swamp of Fallgrim is not the easiest place to navigate.
With four shells and five weapons to find and unlock, there's plenty to get started. As you progress through the story, you'll make your way through three instanced areas that contain dungeons, all of which differ significantly in atmosphere, size, and enemies. Not only that, but once you've completed the full bosses and mini-bosses in each dungeon, you need to return to Fallgrim on foot. There is no easy solution here. More is waiting for you after the last of the three main instance areas, but to avoid spoilers you'll have to figure it out for yourself.
As such, the mileage you can expect from Mortal Shell depends on a few factors. It's a more accessible game than Dark Souls, with more tools that provide a more forgiving combat experience. If you regularly play FromSoftware titles, you will likely find yourself browsing Mortal Shell faster than your average gamer.
During my reading of the game, I managed to complete the game in just over 15 hours. However, I would consider it a slightly faster race than most people will experience. If you want to experience all that is on offer, maximizing each shell you will be able to time between 20 and 25 hours in Mortal Shell. If you choose to take the game's challenge of completing it without a shell to work towards a Platinum Trophy, you can potentially double down this time around.