We had four hours of exclusive hands-on time with Ground Zeroes, which we used to play through the main mission and experiment with extra modes.
Source: March Cover Revealed – Metal Gear Solid V
Edit: Game Informer has been contacted to ask if they only meant they had 4 hours to work with, but they're actively tweeting/retweeting and not replying yet. Some editors also post here so we'll see if they shed a clarification in the thread.