I think it's largely determined by the amount of non-perishable food in the house and number of occupants.
I have circa 200 lbs of "non-perishable" food at any one time (I rotate stuff through the stockpile) , have everything on the list except the leather furniture and the antibiotics, and there's only one person living here. I also have well in of 100lbs of frozen dead animals in my freezers.
Leaves me a result of 493 days.
They forget the most important factor of all though.
Toilet Paper.
I only have a month of so supply of THAT stuff, and once it runs out : I'm making a run for the store
