85 hours in and I am not even close to the finish! Yes, Elder Scrolls IV: Oblivion is huge! I started playing the game in Jan, after over a month (and 85 hours of playtime) I 'm still at it.
Don't let that fool you. The exact game and storyline is really quite short, but there are just that many side-quests in the game. The countless side-quests will have you going all over the place and the stories are exactly better than the main quest. I especially enjoy the Dark Brotherhood quests. In the game, you can join the various factions/guilds in the world and complete their quests. The Dark Brotherhood is the assassins' guild, and it is an evil guild. You know in other games, most of the time you can only join the good guys or they are just a misunderstood faction who are trying to do good or they are a Robin Hood type faction...not these guys. They are EVIL! More importantly, they are FUN to play.
There are 4 major factions in the game, and I have just finished 2 of them. Guess that is another 85 hours.