This is really grim, but it's worth mentioning. I've discussed this in detail at the Last minute tips for parents topic.

Assume the following parameters (and you know what happens when you assume anything... :)

1. Most Americans do not have any practical skills for survival.

2. Most Americans have less than two weeks of food at home and that's counting the freezer mostly. In the event of a major power disruption, most of that would be lost. It's a transition in our meals from canned goods to fresher tasting frozen food.

3. Most Americans not only cannot find potable water, but also don't know how to gather and purify it. If there was no water utilities, many would die in three days.

4. In a true collapse, then money might be valuable only for the first 48 hours. After that point, it won't be accepted for trade items i.e. gasoline, food, ammunition, camping gear, etc. Likewise, ATM cards, Food Stamp Cards, and credit cards will not be worth anything, for no retailer will know if those bills will be electronically paid in a major power outage. As such, places like supermarkets, gas stations, pharmacies, etc will all be complete chaos.

5. Because of the concerns about security, we've seen an increasing militarization of law enforcement. We've also seen huge ammunition purchases by the federal government. We've also seen huge MRE orders as well as checking on the inventory of those companies who deal with MREs.

Now add up these five points. Pause and think about them. If the average person doesn't have skills, supplies, seed, and spirituality, and they cannot rely upon government services in such an emergency to deliver them, then many people would die in the first 72 hours from dehydration from lack of water.

Human beings can live for six weeks without food under certain unusual conditions. Much of that depends upon deep cold, lying down almost in a stupor, and preexisting obesity. For the rest of us, miss three weeks of food, and our blood sugar is too low to sustain life. We'd not have enough energy to find firewood or water.

Under that kind of collapse scenario, then those with three months of food and water could outlast everyone else. That is entirely contingent upon living in a highly remote rural location and likely a newly formed community (which I call a tribe for lack of a better word).

While food supplies would vanish in the first several days of a major SHTF scenario (like an EMP attack), there would still be ways to locate food. See my Last Minute Tips topic that discusses all of that.

If you live in a heavily urban environment, such as the NE Atlantic Seaboard, or are one of the highlighted places on a map link I posted within the last week, or live West of the Mississippi in a drought region, then we could see a huge die of humanity.
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Now what's far more practical is having a three month supply of food and water in case of a weather related disaster, loss of employment, having to support unemployed family members, etc. Then that acquired cushion would be no different then what your great-grandparents ordinarily stored in canned goods in their pantry. (With the exception of the water for many communities relied upon well water and so had a readily available source).