I think you could easily support one person for their natural life. Take flour for example. One 5 pound bag of flour is about 10,000 calories, or enough food for 5 days. A typical store is going to have 50-100 in immediate stock. So that’s a year on flour alone. Do that for all the staples like rice, beans, pasta, and sugar, and you should easily reach 15-20 years of food life.
Then we can move on to more processed things like instant oatmeal, pancake mix, cake mix, pop corn, and things along those lines. There’s probably 100 boxes each of those. That should be another 10 years worth of calories.
Moving on to processed food, there are boxes of cereal, snacks, premade cookies, cakes, chips, and a couple aisles worth of food. Two bags of pretzels is a days worth of calories. These foods probably won’t ever go bad, though they probably won’t taste very good after a decade. This section, again, probably has enough calories for 15 years.
The frozen section will last a while, and has a ton of calories in it. Think about it. It’s densely packed, and a good 50 times larger than your fridge/freezer at home. Spoilage is an issue here, but I’d say you could manage 3-5 years on this section.
So that’s 45 years worth of stuff. I haven’t even considered things like cheese, canned soup, candy, booze, soda, or high calorie food precursors like crisco or vegetable oil.
IMHO the amount of food isn’t a problem. It’s getting it to last, which becomes a crap shoot after a decade or so.