This is obviously a rough estimate of times, but it gives the best guide on how to prepare for unusual events or disasters. You can live three weeks without food, three days without water, three hours in severe weather without good shelter, and three minutes without air. To assess these threats, you examine your local area, terrain, environment, climate/weather, any activity that involves handling or transporting dangerous chemicals or substances and the population (two or four or more legs, how aggressive and territorial, and how many).
Making plans to deal with a threat that contaminates the air would take priority, as would anything that interferes with your ability to breath. These would include a exposure to chemical weapons, an industrial accident, or the risk of drowning. A protective mask approved by Occupational Safety & Health Administration for commercial airborne hazards, a military surplus protective mask or SCUBA gear will address this problem satisfactorily.
The main severe weather threats are wet, and cold. Coats, gloves, socks, underwear, headgear and footgear made out of polypropelene is the best for keeping you dry. Poly-pro wicks moisture away from your skin. When cotton gets wet, it loses all insulation ability, and dries slowly. (However, in hot weather, cotton is ideal, as it readily sheds heat) Gore-tex is a good material as well, when looking for insulated footgear. A simple poncho or a plastic top and bottom is good for shedding rain. Have a few changes of clothes (pants, shirt, socks and underwear and a spare pair of footgear).
Wind is dangerous as well. Wind takes the heat from exposed flesh and can cause frostbite. Cover flesh when appropriate, and take shelter behind, or construct out of limbs and branches, a windbreak.
Canteens can be purchased quite inexpensively, when considering water supplies. Water purification kits (in most general stores) will be necessary when you run the canteens dry, but have a body of water of dubious quality nearby. Many plants are a good source of moisture, such as certain examples of cactus and coconuts. Having the ability to catch rainwater can be essential.
When considering food, there are many options. There are commercially available dehydrated foods (which need boiling water) available, as well as civilian versions of Meal, Ready to Eat (MRE). Multipacks of crackers are a good source of carbohydrates. Knowing how to hunt, what to hunt and how to dress and cook the kill can come in quite handy. Vegetation not toxic to humans can be used as well. Information about local edible plants can be found at a nursery.