Excluding the perishable items I buy weekly (which is about a dozen eggs, 1/2 - 1 gallon of milk, 1 to 2 gallons of fresh OJ, and bread every other week),it could be several weeks/few months. Though since I usually also need to refill my truck with gas on a weekly basis I generally buy a week (or week and half) worth of perishables and restock any long-shelf life items I've used in the last week.
Haven't quite figured out a practical method of going longer on with the perishable items since buying a cow, and having chickens isn't exactly an option at this point due to other considerations....