If you estimate that each play probably lasts an average of 5 seconds and there's a 40 second play clock...that would come out to about 5 seconds out of every 45 seconds with the ball being live. That comes out to 6.666667 minutes per game....so somewhere around 6 1/2 minutes.
But then the play clock usually doesn't run 40 seconds and a team running a 2 minute drill at the end of the game will change things, too...I would guess that it's about 10 minutes.
