The Supreme Court has a scheduled break until 19 Feb.
So she likely has at least that time as a grace period, without having to announce anything. It is common for Justices to be given a lot of deference toward the end of life.
I don’t expect her to volunteer to leave unless the other Justices jointly request it (the custom of the Supreme Court). The other Justices are unlikely to request it, if it will be a matter of just a few months.
If her pancreatic cancer metastasized to her lungs, it is likely a matter of weeks or months, even without pneumonia.
Ussc researching definition natural born citizen