The length of time it takes to heat up a solar shower depends in part on how hot the day is and how much water you are trying to heat. In general, it’s best to give a camping shower at least three hours to warm up in direct sunlight before use, more if you’re looking for an especially hot shower or if the day is overcast. Before showering, it’s always a good idea to check the temperature of the water to ensure it isn’t too hot.