Question

When is the end of the world?

Answer 1

We don't know.

Explanation:

Any number of things could end all life on Earth.

Nuclear War.
Disease.
Asteroids.
Volcanic Eruptions.

The list can go on and on.

Even if all life on Earth were to vanish, the planet itself will remain.
What we do know with a large degree of certainty is that in a few billion years, our sun will deplete it's hydrogen and begin fusing helium into carbon causing the sun to balloon into a red giant and possibly swallow the planet. This could be the true end of the Earth.