I'd say about 5-6 hours but games seem to get smaller every year so i wouldn't be too surprised if it was something like 3 hours long.
End of 2012 prediction:
xbox 360 : 73-75 million playstation 3 : 72-74 million wii : 104-105 million
Most hyped for :
Bioshock: infinte, The Last Of Us, Alan Wake's American Nightmare and Agent