How long do you survive without warmth?

Heat is in some sense anything over Absolute Zero. So humans always have some heat available.

That said, the most extreme temperatures humans have lived in without our modern heat sources have got down to around sixty below zero degrees Fahrenheit. Bundled up in furs, within an enclosed area, and with themselves for warmth, Native Alaskans and Native Canadians did fairly well.

They did have the heat source of themselves, and that is 98.6 degrees Fahrenheit each, with much of that heat trapped by the clothes and the shelter.

Without clothes to trap our own heat, and no shelter, we cannot survive long. Fully dressed workers in Canada are advised to stop non-emergency work if the temperature is 45 degrees below zero, and even at 40 below zero, may only be out for a half hour.