This depends on what kind of player you are. If you are going to just steamroll through the main story, you could beat the game in about 30 hours or so. It's a long story and is definitely enough to get a great experience out of the game. If you're going to be exploring and joining different Factions and taking on side quests, though, expect to spend up to 90 hours in Cyrodiil and far more if you want to complete everything there is to do in the game.