There is no way to determine the average amount of sunlight for all deserts on the planet because there are multiple types of deserts and the location of the desert plays a part in how much sun each one gets. A desert like the Sahara Desert would get more sun being at the equator than a desert that was located more north or south.
Weather and season can also affect how much sun a desert gets. In the winter, the days are shorter thus giving deserts less sun exposure during the day. The exact opposite happens in the summer when they get more hours of sunlight per day. The amount of rain in a desert is low, but it can fluctuate from year to year changing the amount of sun as well.