That's just for running through the story without tackling any sidequests or factoring in replay. There will also be console-exclusive missions, and everything that's included could add up to around three or four hours of content.
We'll get to play Ground Zeroes when it launches in North America and Europe on March 18 and 20, respectively. It'll run you $20/$30 (digital/physical) on PlayStation 3 and Xbox 360. The PS4 and Xbox One versions will be priced at $30/$40, with the added bonus of saves that will in some way work in conjunction with The Phantom Pain.