Can a doctor say exactly how long someone with advanced cancer will live?

No. The National Cancer Institute is direct about it: "no one knows the future. Predicting how long someone will live is difficult."

A doctor can give you an estimate. It draws on the type of cancer, the treatment, past illnesses, and other factors. That makes it an informed guess. It does not make it a measurement.

The system itself expects estimates to be wrong

Here is a concrete example. To enroll in hospice, a doctor must certify that a person is expected to live six months or less. As NCI puts it: "Hospice care can be continued if you live longer than 6 months, as long as your hospice doctor recertifies your condition."

That clause shows an estimate is not a deadline. A complication such as an infection can also change things.

Better questions than "how long"

If you want information you can act on, ask differently.

Ask what would change the answer, in both directions. Ask what to expect over the next stretch, so the next change is not a shock. And ask directly whether anything is time-sensitive, meaning something you should do sooner rather than later.

You are also allowed not to ask. Some people find a number unbearable and would rather not carry it. If that is you, say so plainly, and say whether someone else should be told instead.

What is worth doing either way

Some things make sense whether the answer is six months or six years. Completing advance directives, naming someone to speak for you, saying the things you would regret leaving unsaid, and taking the trip while you can still enjoy it.

None of those require knowing the number. That is the quiet advantage of doing them early.