Another way of looking at it is how long are you prepared for them not to be open. I live in central Washington, so not many disasters, however the County plan urges all residents to be self sufficient for 3 days. Talking to Old Timers the said back in the early 90s the highways were closed due to a severe snow storm and things got very thin after 6 days.
i was in S.C after Hurricane Hugo and one of the great things was the block parties after a couple of days, when everyone cooked what was thawed from their freezers.