Approximately how long does it take the increased ultraviolet and X-ray radiation from a solar flare to affect radio propagation on Earth?

Explanation

Ultraviolet and X-ray emission from a flare is electromagnetic radiation, so it crosses the Sun-Earth distance at the speed of light. One astronomical unit is about 150 million km, and 150,000,000 / 300,000 km per second is about 500 seconds, or roughly 8 minutes.