The map is open but divided into regions, with each region feeling like a self-contained story to some extent. In the first two regions I visited, I took my time, explored every nook and cranny and shiny object that piqued my interest. Before I knew it, 30 hours had flown by, and I was only 30% done with the main story with a review embargo date fast approaching. I didn't want to, but I had to force myself to be more disciplined in the mid-to-late game and ended up at the 47-hour mark by the time I finished the story. Which is to say that I really have no idea how long the main story is on its own—maybe 25 hours—but my advice is to not worry about that. Play Ghost of Yotei the way you want to play it and enjoy it. Follow what interests you and good luck trying to not get distracted two or three times on your way to the next thing.