I'm not sure exactly how long it was, but it took me about a week of playing ~6 hours or more a day to beat the main storyline, do all the Assassin contracts, and collect the majority of location-based collectibles like chests and shanties and Animus fragments. That includes hunting, whaling, and pirating to get the upgrades needed to finish. Not to mention all the modern day stuff plus watching videos and listening to audio clips. And even now I've still got a couple of the legendary ships to still beat, some buried treasures to find, and a few other minor things to do. Overall it's no Skyrim but yeah, there's plenty to do and a good time investment needed to do it all. And yeah, the trailers were good for hype and showing off some of the cooler, flashy moments, but they really missed a lot of the deeper character interaction and the striking, near decade-long character arc that is the backbone of the game's story.