According to director Yuya Tokuda, it should take an average player "about 40 to 50 hours" to finish the Monster Hunter: World story mode.
In an interview with the German game website Mein-MMO.de, Yuya said, "Depending on the type of player, I suppose it will take about 40 to 50 hours to clear the story[.]" Yuya did add that, "it wouldn't be a Monster Hunter title if there wasn't content to enjoy post-game. Players can continue to forge and upgrade their favorite weapon and armor after the story is over."
Also, executive director Kaname Fujioka added that there will be weekly event quests and free updates that will further beef up Monster Hunter: World content-wise post-launch.
According to howlongtobeat.com a typical Monster Hunter game usually reaches into the hundreds of hours to complete fully. However, Tokuda refers only to Monster Hunter: World's story mode, so there's a chance that the new game could stretch closer to a hundred hours for a full completion of the game. Even then however 40 to 50 hours is still more than enough for any player to spend on a story mode.
Monster Hunter: World was announced at this year's E3 and just wrapped up a closed trial. An open beta for the PlayStation 4 is scheduled for December 22 to 26, with the full game coming to PS4, Xbox One, and PC on January 26, 2018.