User blog comment:JakePT/Mass Effect 3 to be Revealed at VGA Awards?/@comment-3101247-20101122140510/@comment-217683-20101125013536

Also, did a bit of looking about ODST, and you are correct that it covers more than six hours. In fact, as it just so happens (I love coincidences like this) the first playable level actually occurs six hours after the opening cinematic. So yeah, given that you don't even start playing till 6 hours after the events of the game begin, it is a fair assumption that the events of the game take more than six but less than 48 hours to transpire.