In the calculation to figure out how long it would take the Sun to cool I had used Fourier’s law of thermal conduction. If fusion stopped, we’d be there in a little over a million years. So all together, if fusion were to stop today, the Sun’s surface temperature should be about . It doesn’t take much to end all life on Earth, so we would probably have, at most, a couple million years. It would take a little over one hundred thousand years for the surface temperature of the Sun to drop by 1%.