https://www.gamespot.com/articles/assassins-creed-odyssey-how-long-is-this-game/1100-6462176/

Assassin's Creed Odyssey is the largest game in the franchise to date. The overall map in Odyssey is over twice the size of the one in Assassin's Creed Origins, and it's filled with a branching main storyline and hundreds of side quests. Simply playing through the game's campaign only scratches the surface of how much you can do.

For our Assassin's Creed Odyssey review, editor Alessandro Fillari completed the game's main story in just over 50 hours. And that's without hitting every single island and side mission available. There's a broad array of optional content to tackle outside the main campaign, including additional islands to explore and mythological creatures to slay.