Game Informer has a MGS blowout for their March edition and talked about Ground Zeroes. They had an exclusive play of the game and they finished the main mission along with extra modes in about 4 hours.
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.
UPDATE: Judged just by the core story mission, Ground Zeroes is short," Game Informer writes in the cover story (only available in the magazine, not online). "We completed it in just less than two hours
At first thought they meant they only had 4 hours with the game but they state they got through the main mission in that time. Kind of disappointing considering the price point for GZ
Let's not forget that this is just the intro to MGS V (the phantom pain) and it costs $30 retail for PS360 and $20 digital but $40 PS4 and X1 retail and $30 digital. Not only that but Kojima has said if Ground Zeroes doesn't do well that they will delay MGS V.
Kojima has defended the length of the game but made things slightly worse by detailing what was actually in the game. The main mission, 5 side missions and a time trial like mode