We likely have much less than 2 billion years before the earth becomes uninhabitable: https://en.wikipedia.org/wiki/Timeline_of_the_far_future
-- Karl Popper
Loves this one for example:
15,000 yrs from now: According to the Sahara pump theory, the precession of Earth's poles will move the North African Monsoon far enough north to convert the Sahara back into a tropical climate, as it was 5,000–10,000 years ago.