Infinity Ward's Robert Bowling has said that Modern Warfare 3 single player campaign length completely depends on game's story. In an interview to Gamefront Bowling said, “Modern Warfare 3 is big,”
“It has that epic single player campaign and it’s all about taking players places they’ve never been before.
“… Our philosophy on the campaign has always [been] the same. We never set out … to make X hours of game time. We write a story and the story dictates the length.
When quizzed, So how long the campaign will be then?, Bowling replied, “The game’s not finished yet. I can’t say it’s X amount of hours cause we don’t know.”
However Robert Bowling did mention the fans of the franchise won't be disappointed.
“Coming out of Modern Warfare 2, we did have this very big vision, and a lot of that was a by-product of having the 25, 30 million players that we have.
“So Modern Warfare 3 is about the fans – in every aspect of the game.”