It will take you around 13 hours to complete Doom's campaign, according to developer id Software. The game's official Twitter account mentioned the number recently, further explaining that the difficulty level you play at will be a determining factor in how long it takes you to complete it (via IGN).
.@Unreal389 The difficulty level plays a factor, but folks playing at the office average 13+ hours in the campaign— DOOM (@DOOM) February 11, 2016
Doom's campaign isn't its only offering, however. It also comes with a multiplayer mode, which, for those interested in it, should extend the number of hours played by quite a bit more.
13 hours is above what shooter fans have come to expect, as anywhere in the 6-10 hour range is pretty standard. Of course, your play style and difficulty selection will determine how slowly or quickly you progress through and complete a game.
For more on the topic of game length, check out GameSpot's editor roundtable on the subject.
After years of development, Doom launches in May for Xbox One, PlayStation 4, and PC. A beta for the game will be held ahead of launch, but id Software has not yet said when this will begin or what it will include.