Actually Stephanie, there is more detailed information at
http://ready.gov/
I don't think the duct tape and plastic sheeting were suggested for long term survival. You would run out of oxygen eventually, but in the event of certain attacks and if sealed properly, it would protect you for several hours before you ran out of oxygen. By then, the biggest threat may have subsided.
I think you would need to have your plastic sheeting cut and ready to go to make sure you can get it sealed in time. I doubt too many people are actually taking these precautions. We may be sorry later though. :o