Metal Gear Solid: Ground Zeroes is a $20 prologue to Metal Gear Solid 5: The Phantom Pain. The budget price should have been a clue as to how limited the experience might have been. According to a recent playthrough of the game, the main campaign can be completed in two hours.
According to Kotaku, Game Informer's preview notes a two hour completion time for the campaign. However, this doesn't include any side quests.
Series creator Hideo Kojima said that the reason to split up MGSV was ease players into the experience of playing, with Ground Zeroes serving as a "tutorial" for the game. The Phantom Pain will be "hundreds of times larger."