halacha - How hungry does a Jew have to be to eat non-kosher food?

I understand that Jews are allowed to eat non-kosher food in order to save their lives.

My question is, how long does a Jew have to be deprived of food, before he or she is allowed to eat non-kosher food?

For this question, I'm assuming that the hungry Jew is a full grown adult, with no chronic health problems.