-Three hours without shelter.
Maintaining your core body temperature of 98.6 degrees more or less is of the utmost importance. In my opinion understanding how body heat is lost or gained in it's most rudimentary form is crucial so that it can be controlled(as best as you can) rather quickly.
-Conduction: Heat transferred (either lost or gained) when in direct contact with another object of differing temperature.
In the barn in the winter if you watch the cats they avoid the cold concrete like the plague. When they stop or sit it will be on anything other than the floor so that they are insulated from it to minimize their heat loss through conduction. In contrast our dog in the summertime will flop down on his side and take full advantage of the cool concrete trying to conducting heat into the floor to cool down.
-Convection: Heat transferred (either lost or gained) by currents of air or water. Staying out of the wind will conserve body heat. Whether that be shelter or wearing wind proof clothing or improvised articles of clothing, garbage bag, poncho etc. In contrast a gap in the mountains will provide a cool breeze as the cooler air sinks and flows to the bottom.
-Radiation: The heat coming from your body is a form of radiation, insulating layers of clothing, shelter or reflective (space blankets) to reduce your heat loss. Also using the radiation from a fire or the sun to your advantage when it is cold or avoiding it is hot. An interesting not is that when the temperature was in the single digits recently our darker colored cows would stand out of the wind(convection) and in the sun(radiation) and their hide felt warm to the touch as opposed to the lighter colored cows. The color of your clothing can be used to your advantage to gain radiative heat or reflective to avoid it.
-Perspiration/evaporative cooling can be used to your advantage to stay cool and avoiding perspiring or getting wet when it is cold.
-Respiration : Heat loss can be manipulated to a lesser degree by breathing warm air though an article of clothing or keeping your face covered.