Err.. well, if by "beat" you mean main story line, then prolly several hours (if thats all you plan to do).. or you can go around the map exploring the hundreds of dungeons, side quests, and faction quest for countless hours.
This was my first game for my 360 back in 06... I still have the same disc, and I have yet to feel like there is nothing to do in this game.
Also, the game doesn't "end" once you beat the main storyline, you can still do whatever you want. --- "Mmm.. This IS a tasty burger." - Pulp Fiction <<>>