Techland made waves recently with a certain claim about 500 hours but they have now cleared it up and given more concise approximations on what you can expect form Dying Light 2.
Video games that take 500 hours to complete probably sound ridiculous to the average gamer because they would assume that's how long it takes to get through the quests, missions and storyline they offer. As such, potential players were dumbfounded when Techland announced Dying Light 2 would take that much time to complete.
The core of the misunderstanding here was that Techland spoke about literally completing everything in the game, including full map completion and grabbing all the collectables. When you factor such menial tasks in, there are countless games that reach the 500-hour milestone and if you add grind, you can easily reach into the thousands, depending on how much of a completionist the player is.
Anyway, this was followed up by a clarification from the devs who posted info on how long it will take to complete various goals in the game. For example, if you're in it for the main story and all of the sidequests without chasing collectables, you are looking at 80 hours of gameplay.
If you only want to experience the story through the main missions and ignore the side quests, the number of hours comes down to 20, which is still quite chunky, considering you are only going through the skeleton of the game.
You can probably go into the side quests as much as you feel, which would mean the expected gameplay time can be variable, depending on the amount of content you decide to consume.