It wouldn't even take 72 hours.
As soon as a few observant people realize it's an emp within the first few hours (independent electronic systems would fail simultaneously, electric grid, cell phones, utilities, modern cars etc, unless electronics were protected in a faraday cage) the observant people who understood the scope of the disaster would be looting within the first 6 hours to start hording supplies then when panic sets in the masses would be looting by the end of day 1.
But whatever I'm getting way off topic here
Edit- brining up EMPs is critically important because I feel it is one of the single biggest threats we gave that the average person is totally unaware of.