Thanks to a image posted on Gamefaqs earlier today, it seems someone has already beat Metal Gear Rising: Revengeance and did so in only 5 hours and 34 minutes.Before you get too worried, the stats seem to indicate the player was rushing to the end, so this could be something of a speed run. By the same token the stats could indicate the person playing wasn’t good at Revengeance, which might mean the game can be completed much faster.
Honestly this shouldn’t come as a surprise to most, since Platinum Games isn’t known for making long games. Madworld could be conquered in an hour or two, Bayonetta gave you a bonus for doing it under three hours, Vanquish was also about five hours and Anarchy Reign’s could be done in less than three hours. Obviously this time also doesn’t account for the VR missions (Sony gets an additional 30) or any of the side objectives in Revengeance, so you should still be able to get your moneys worth.