3 to 3.5 hours, barring overtime. There is an occasional 4 hour game.
There are WAY too many stops in the game. It's not the game that's at fault, it's TV. If there's one thing I like about soccer (futbol), it's that there aren't any stoppages.
Unless my team is playing at the time (NY Giants - not so good so far), I watch a channel called NFL Red Zone. There aren't any commercials and they switch to the most exciting game at the time. Sometimes they show two, three, four at once if they need to. They'll also do rewinds of plays that led to the current excitement. It's the best way to watch the game(s) if you don't have a current rooting interest in one team.
National Football League: NFL RedZone Every Touchdown From Every Game