There is a show on Outdoor Channel called Best Defense: Survival. They have a lot of good advice, I recommend it for people that are concerned about the SHTF scenarios.
They broke it down into a few situations but they recommended that you store enough food/water for a 90 day situation, because somewhere between 7 to 90 days is the most likely scenario.
I watched it last year and they have some interesting ideas. Have you ever noticed that Michael Bane has some sort of a tic once in awhile? I got a laugh out of Michael Bane when he went to a hand gun manufacturer. Michael and the owner of the plant were talking about the hand gun and the owner accidentally pointed the gun at Michael. Michael got this pissed look on his face and grabbed the guy by the wrist and pointed the gun in another direction. I'm surprised they left that in the show.