Dying Light 2 on PS4, PS5, Xbox One, Xbox Scarlett, and PC only takes 15 hours to beat, but it's also 100 hours long. In other words, there's about 100 hours of content in the open-world, post-apocalyptic game where you parkour on Zombie's heads. However, if you're looking just to beat the main story, that will take you between 15-20 hours. Of course, all of these numbers will vary a bit depending on your playstyle. For example, speedrunners will probably beat the game in two minutes. But players who read every little piece of text and die 47 times a mission, well, expect more than 15-20 hours.
Of course -- and Techland points this out -- it's hard to give an average playtime for an open-world game where there's so many variables at play. In other words, don't expect these numbers to b exactly indicative of your experience. They are ballpark figures.
That said, 100 hours for every piece of content sounds like it may be an estimate on the high-end. Yet again, Techland has said the game is 4x bigger than its predecessor. Speaking of its predecessor, its main story took about 17 hours. So, about the same. However, a completionist run only took about 50 hours. In other words, it's interesting to see no discrepancy in the mainline length, but a huge one in the length of a completionist playthrough.
"It's been 15 years since humanity lost to the virus," reads an official story synopsis. "The last great human settlement exists within an unforgiving, infected world, plunged into a modern dark age. During the day, bandits, factions and starving survivors roam the streets scavenging for scraps - or someone to take them from, by violence if necessary. At night the infected roam free, evacuating their dark hideouts to prey on the living. You are Aiden Caldwell, an infected survivor. Your exceptional agility and brutal combat skills make you a powerful ally and a valuable commodity in this dangerous world. You can achieve things no one else can. Enter places no one else dares. With your unique abilities you have the power to be an agent of change within this decaying metropolis. The fate of the city is in your hands."
Source: Prankster 101