Hey TinouX,
from where you take this numbers? 21h gameplay!
http://www.gamespot.com/ps3/adventure/metalgearsolid4/news.html?sid=6190944&tag=top_stories;story;0
"After the trailer, Kojima showed some live MGS4 gameplay and unveiled new information. The demonstration ran for about 40 minutes with an experienced staff playing the game, and Kojima mentioned that they're showing off only about one-fifteenth of the whole game."
So ... 40min x 15 = 600 min
600 min / 60 = 10h
21h or 10h ?