The solar flares occur suddenly on the surface of the Sun. They can last for only few minutes, or for few hours, depending on their intensity.
Basically, the solar flares are large eruptions o electromagnetic energy on the surface of the Sun, looking like spots from Earth's perspective. These eruptions on the Sun can be smaller and larger, and they travel with the speed of light, so the closer space objects, Earth included, feel the effect instantly once they appear. A large solar flare can actually be very dangerous, as it has the potential to destroy everything electrical on Earth, getting back the human kind back several centuries in a second.
