[rbg81]

How many people would die in the ensuing chaos after an EMP attack? Lots. Law and order would break down. No telling how even solid citizens would behave once they (or their kids) got hungry. And it would take years for our infrastructure to recover. Wouldn’t surprise me if we lost greater than 20% of our population even without nukes.

[Michael Barnes]

Its the "nine meal rule". Could be three days, could be five. But most of America does not have more than nine meals, following that scraps and finally looting and scavenging.
Want to reduce the time to start social decline early? Cut power.

To be honest, I'm not so worried about the cold where I live (it will suck, but we can survive). What I worry about is no power and trying to keep food safe in the summer.

[Secret Agent Man]

We’d lose half our population, at least.

People don’t know how to take care of themselves. They don’t learn anyhting useful or plan ahead. They’ll start killing each other for a can of creamed corn.

Do it in winter and at least half are dead by summer.