let’s say, I have one day, a week or a month left to live suffering from an illness,

A big problem with this line of thinking is that in real life you rarely know this, especially while remaining aware enough to commit to such a plan. It’s just a prognosis, based on averages.

or I know that after a while I’m going to have a heart attack that will definitely kill me.

1. You want to give up your remaining time because a heart attack is in your future? Usually you can reduce the chances with diet, exercise and medication.
2. For many heart attacks, survivability is directly related to how quickly you get medical attention.

Generalizing this back to what I believe is the goal of your question, it’s rarely certain or predictable. When it is, you may already be too far gone to make such choices (hence legal documents like DNR)