We'd be fine for longer than 72 hours. I do a lot of my shopping from Costco so it comes in bulk anyway and if necessary could probably manage at least 3 weeks on what we have. Smaller portions obviously and less fresh fruit/vegetables. We do have both a barbecue and a camping stove so not reliant on electricity for cooking. I do also have some bottled water.
I spent a chunk of my childhood on the RAF base closest to the East German border. My father insisted we were "prepared" just in case. I keep a bag in the very bottom of our boot with a change of clothes for everyone, toiletries, some distractions for the kids, torch, spare batteries, first aid kit, tool kit, cash, sewing kit, medication, blankets, cutlery etc. I check it every 3 months or sooner if we've used something. Has come in handy for all sorts of things.