How long does it take to cook a turkey?

This was our 2nd Thanksgiving that we cooked our own turkey.  I don't know much about cooking turkey, but I do know it takes a long time to cook a 20lbs bird.  We gave ourselves 6 hours and still had to add a couple minutes to make sure no one got poisoned.  Our friends, who will remain nameless (www.jeremynicks.com) thought they could cook their bird in 3 hours.  Let's just say it's a good thing my nameless friend (www.bigcrate.net), is a communications director (www.livingwater.com) not a chef.  We still love him and respect him way too much to reveal his identity.