About 5 hours.
I don't believe that and if you did it's your fault for rushing. I've spent about 15h on normal, doing all the side stuff and I can't wait to start the 2nd run on hard and on the other path, till I get the platinum. It's a 25h+ game for me.
I think the average playtime is 10h, if you look at www.howlongtobeat.com, which is fine. MGS Ground Zeroes has an avg playtime of less than 2 hours.
If you act like Infamous SS lasts 5 hours I can say MGS GZ lasts 6 minutes if you rush, ok?