I've heard ranges from four meals to three days. I suspect the more urban the environment, the shorter the time.

Of course, those of us here are well prepared...?

Britain 'four meals away from anarchy' if cyber attack takes out power grid
https://www.yahoo.com/news/britain-apos-four-meals-away-060000661.html

"a study by the UK’s security services had estimated the country is “four meals away from anarchy” because looting would erupt and civil order would start to break down as soon as people had eaten what they had in their cupboards and fridges."

There are organizations I've never heard of before, i.e. the "Centre for the Study of Existential Risk."

Keep on Preppin'