Approximately how long does it take the increased ultraviolet and X-ray radiation from a solar flare to affect radio propagation on Earth?
Why Ultraviolet and X-ray emissions are electromagnetic radiation, so they travel at the speed of light. The Sun is about 93 million miles (150 million km) away, and light covers that in roughly 8 minutes, so the ionosphere reacts (sudden ionospheric disturbance, daylight-side HF blackout) almost as soon as the flare is seen.
Watch out The 20 to 40 hour figure describes the slower coronal mass ejection, the cloud of charged particles that arrives later and disturbs the geomagnetic field; 28 days is the solar rotation period that brings the same active region back into view.
Light takes 8 minutes, particles take 20-40 hours, the same sunspot returns in 28 days.