There is no simple answer to the question of how common rainbows are, as they can vary in frequency widely depending on the region and time period. To get a general idea of their possible frequency, the conditions necessary to create a rainbow need to be understood.
The two fundamental ingredients to make a rainbow are airborne water particles, such as rain, mist or spray from a waterfall and light, typically sunlight. Achieving this arrangement the viewer must then find a position facing the water, but with the light source behind them. The position must result in an angle between the light ray and the observer of exactly 42 degrees. This may seem restrictive but typically the area where the water and light combine is such that countless various positions exist which provide the necessary angle. Because of this required condition, natural rainbows are most common in the eastern sky during sunset and the western sky during sunrise.