If a star thousands of light-years away ceased fusion now, how long until we see darkness?

For another thousand years until the last photons arrive

The physical cessation of light generation due to internal failure, such as the stopping of nuclear fusion, is completely decoupled in time from the moment observers on Earth witness that change. This is because light travels at a finite speed across space. If a star is located several thousand light-years away, the light currently reaching Earth was emitted thousands of years ago. Therefore, if that star instantaneously stopped generating new light in its core this very second, the existing photons are still traveling across space. Observers on Earth would continue to see the star shining brightly for approximately another thousand years until the final emitted photons complete their journey.