Halo 4's leaked achievement list hinted at only eight missions, but 343's Frank O' Connor has assured gamers that the game is by no means short. The studio has also confirmed that the content delivered via Spartan Ops DLC will be longer than Halo 3: ODST.
Commenting on a GAF thread focusing on the length of Halo 4, O'Conner - using his 'Stinkles' handle - said, "Number of missions has no bearing on the length of the campaign. It's a Halo sized campaign. PLUS Spartan Ops etc."
343's designer David Ellis chipped in under his 'lybertyboy' forum handle, "[Spartan Ops] season alone is longer than [Halo 3: ODST]. Spartan Ops and Campaign are completely separate teams. As Frank mentioned earlier, the main campaign is substantial and will compare favorably with previous entries in the series."