It doesn't take long to survive unless you have what it takes! Despite our point of view, humans are actually quite fragile creatures that live in a narrow habitable area of our planet. The exact combination of oxygen, gravity, air pressure, temperature, nutrients and water must be available and constantly maintained in balance, or we will perish.
However, we are still relatively resilient when in our comfort zone. Left alone with no other resources, the average healthy person could live for weeks with only air to breathe. However, in the presence of weather conditions, stress, injury, disease or danger, access to additional resources is imperative for survival.