What is the halacha on how imminent the danger of death has to be for you to be allowed to invoke pikuach nefesh and break a commandment to save a life?
Obviously if someone is drowning before your eyes and you can save him by breaking a commandment (without seriously endangering your own life), you do it. But what if the danger is, say, exposure to radiation that will cause cancer 20 years later?
I know that in cases of uncertainty on whether a life is really at risk, you err on the side of offering assistance. [Yoma 8:6] But here I am asking about timing, not uncertainty. Any sources?