The next time I go the the USA, i'll eat half a happy meal...
My question is how does one know when the body starts to break down muscle? How long does that take in an "average" person?
A difficult question to answer - in terms of exact figures
A good rule of thumb is when, your fat reserves are gone, your muscle is next.
To check the survival fat reserves, check the abdomen. Do a pinch test about the waist and then about the triceps (back of upper arm) fold. Compare this at the start of your survival situation and check progressively as you go along. Once there is no more fat to pinch, you're almost down to lean meat alone. That's when protein and calorie intake will prevent you from tipping over to severe muscle breakdown.( reduced muscle size and increasing muscle weakness)
Reserves can last from anywhere up to a few weeks. metabolic rate, environmental temperature, activity etc all make a difference as to how long the reserves last.