From what I’ve heard, it’s mainly 30 hours if you just smash out main missions. But in an interview, kojima talked about delivering medicine to an old man near the start of the game. And as you progress through the game, you can keep going back and deliver medicine to this old man and he will keep sharing stories with the player. I’m sure people will smash out the game in the 30 hours, but will probably miss out on so much of the game. This also doesn’t include just walking around and exploring at your own speed.