It is not easy to give precise heating times: this depends on the sun's rays, the way in which the shower is laid out and the initial temperature of the water.
As a rough guide, here are the temperatures observed after leaving the shower in the sun for an afternoon (outdoor temperature of 81°F):
In 2 hours, the temperature increased from 75°F to 93°F
In 3 hours, the temperature increased from 75°F to 100°F
To heat it up more quickly, lay the shower down to increase the area exposed to the sun.

Do not leave your shower behind the windscreen of a car throughout the day in full summer to speed up the heating of the water. Our tests showed that this can heat up the water to a temperature of 176°F. Such high temperatures can damage the components of the shower, making them age prematurely, and, above all, be hazardous for the user.