Answer: 24 hours
This is because June 21st marks the astronomical start of summer for the northern hemisphere with the summer solstice. During this season the days are longer than the nights, and as we get closer to the North Pole, we will have up to 24 hours of sunlight, a phenomenon called the Midnight Sun.
This is possible because during the summer solstice, due to the tilt of the Earth's axis, the North Pole is closer to the Sun than the South Pole. However, paradoxically the Earth is at its furthest point from the Sun (aphelion).
That is why on June 21st, the sun is visible for 24 hours and the next days (the closer it is to the north pole) it will stay that way. Therefore, the inhabitants of this area can enjoy sunlight for a longer period.