Doom: The Dark Ages offers a larger campaign experience compared to its predecessors, with 22 missions that vary in length and structure. While the typical chaos of modern Doom games is present, some chapters feature open objectives, while others introduce new mechanics and enemies.
The game’s pace can be challenging to determine, as each mission can last anywhere from 30 minutes to several hours. For example, Siege Part 1 takes around 10-15 minutes to complete, but other missions like Ancestral Forge and The Forsaken Plains span over an hour.
A standard playthrough of Doom: The Dark Ages takes approximately 4-6 hours longer than its predecessor, despite having nearly double the number of missions. Completing the campaign with all secrets and collectibles can take up to 30 hours or more.
In comparison, reviewer Morgan Park completed the game in around 16 hours, finding most of the secrets without extensive searching. However, for those aiming for completionism, scouring every level can be a time-consuming task.
Source: https://www.pcgamer.com/games/fps/doom-the-dark-ages-missions-list-how-long