How long does it take a coronal mass ejection to affect radio propagation on Earth?
Why A coronal mass ejection is a cloud of charged particles (plasma) physically blasted off the Sun, so it travels at the speed of the solar wind, not the speed of light. Typical CME speeds put arrival at Earth anywhere from about 15 hours for a very fast event to several days for a slow one. When the plasma hits Earth's magnetic field it disturbs the geomagnetic field, raising the A and K indices and often degrading HF paths, especially polar ones.
Watch out The 4 to 8 minute figure is the light travel time from the Sun to Earth, which applies to the X-ray and UV burst of a solar flare that causes sudden ionospheric disturbances, not to the slower particle cloud of a CME.
Light takes 8 minutes, particles take hours to days. CME = matter, not light.