How many days are there in summer?

The average length of the northern hemisphere summer, calculated as the average time from the June solstice until the September equinox, is about 93 days and 15½ hours.

The average length of the southern hemisphere summer, calculated as the average time from the December solstice until the March equinox, is about 89 days.

(The reason the southern hemisphere summers are shorter than the northern hemisphere summers is that orbiting objects travel faster the closer they are to what they're orbiting, and Earth is closest to the sun in early January and farthest from the sun in early July.)