While GTA V wasn't the longest game at the time of its release, there have been other RPG action games like Witcher 3 and Elder Scrolls: Skyrim clocking in massive hours of playtime.
GTA V is still quite a large game, where a player can spend countless hours exploring all its secrets, easter eggs and many fascinating characters spread across a giant map.
The story alone can take up a massive amount of time to complete, while finishing the story missions along with side missions can increase playtime significantly.
However, the true way to experience GTA V is to explore as much of it as you can, and complete all the side missions, as the game is significantly more immersive and entertaining that way.
You could sprint through the story and stick only to doing the Story Missions, but then you will miss out on a lot of quality content that GTA V has to offer.
GTA V play time: How long does it take to finish the game
Example of a completionist run:
There are several ways to go through the game. You can either just skim through GTA V, playing only the Story Missions and ignoring all Side Missions and Collectibles, or you can go the Completionist route.
The latter way requires you to play all Side Missions, find all Collectibles and any other such offerings. It will add significantly more time, but it is the only way you can complete the game 100 percent.
Main Story: 31 Hours
Main Story + Extras: 49 Hours
Completionist: 79 Hours
GTA V's single-player campaign's 31 hours is a respectable as a lot of other games, and provides enough value for money to justify the game's AAA price-tag. A lot of games from that time did not do this, failing to provide enough interesting content to justify a $60 price tag.
Also read: GTA Online Collectibles: Full list of all collectibles