Yes, around one hour. Two hours if you really take your time and listen to all of the tapes/look for secrets. There's side ops too that I haven't tried yet. But they're on the same map. Ground Zeroes is essentially the Tanker Chapter or Virtuous Mission of MGS5; the prologue chapter, but it's far shorter than those were, imo.
It's been released since Konami need the money since their release schedule is staggering badly and MGS5 is a ways off (Q1 2015 at the earliest, supposedly).
I'd say it's absolutely a rental or one to wait for a price drop for. Or just wait for Phantom Pain (the real MGS5) to release, I think GZ will be a part of the game then anyway. $30/£25 is too much for it. But it is incredibly fun and heartwarming to see Kojima Productions keep the old school MGS flavour alive.
Also, as a side note, I think Kiefer is good as Big Boss [you have to listen to the tapes to hear it though, there's only three cutscenes].















