Two weeks
Explanation
The Joke
A doctor examines a patient and delivers the news: "It's quite a bad hangnail, Ms. Derkins. You have two weeks to live." The punchline appears in the caption below: "Those comets that are about to hit Earth have made prognosis a lot easier."
The joke is that the doctor is giving a seemingly absurd medical prognosis -- two weeks to live from a hangnail, which is an extremely minor ailment. But the caption reveals that the "two weeks to live" has nothing to do with the hangnail at all. Comets are about to destroy the Earth, so literally everyone has two weeks to live regardless of their medical condition.
The Humor
The comic uses misdirection brilliantly. The setup looks like a standard "bad doctor" joke where an incompetent physician gives a wildly inappropriate terminal diagnosis for a trivial condition. The reader assumes the humor is about medical incompetence. But the caption completely reframes the scene -- the doctor is technically correct; it's just that the prognosis applies to everyone on the planet equally. The additional layer of humor is that the doctor is apparently still seeing patients and going through the motions of medical practice even though an extinction-level event is imminent. There's something absurdly professional about diagnosing a hangnail when the world is about to end.