Glen Schofield, one of the founders of Sledgehammer Games, has confirmed on Twitter that the campaign coming in Call of Duty: Advanced Warfare will be longer than those that have appeared in the most recent entries in the series, or as he puts it himself: "Coming in longer than the last few."
Since an experienced player can easily plough through the most recent story mode in approximately four or five hours, we don't expect this means we'll get a campaign lasting ten hours or more. However, that's just guesswork on our part, and we'll get the answer one way of the other when the game is released on November 4.