On this page of the Diablo Immortal guide, you will find information about the length of the game. You will learn how many hours it will take to complete the main story, as well as how much time you have to spend on reaching the maximum level and Paragon levels.
If you want to play Diablo Immortal solely for the story, it should take you around 25 hours to complete all story quests. Of course, this time may vary slightly depending on how much time you spend customizing your character, choosing your skills, etc.
It should also be noted here that, from time to time, players will be forced to reach a certain level of experience before they can continue the story of Diablo Immortal.
Paragon levels and side activities
In addition to story missions, Diablo Immortal has much more to offer, e.g. Paragon levels, Elder Rifts, Challenge Rifts, Dungeons, completing Bestiary, obtaining Achievements, World Bosses, Bounties, and other PvE and PvP activities. If you want to reach the maximum Paragon level, get everything in the Codex and Bestiary, and arm your hero properly, you'll spend up to 100 hours or more with Diablo Immortal.