This video interview conducted by Grey Viper at E3 discusses Battlefield 3 with DICE's Patrick Söderlund. They discuss various aspects of the game, and how it expands and improves upon previous games in the military shooter series. Along they way they ask how long the single-player campaign is expected to be, and received the following answer:

I don't have a number for you... yet, 'cause the game is not done yet, and I don't want to... To me the game can be anything from six, seven, eight, to ten hours, depending on what we net out once we're done. I think the most important thing for me is that the game delivers upon a consumer experience that we're happy with, and then, that will then... that's how we look at it, and that will tell how long the game is.