Not as often as you think.  Rain is naturally soft water with few, if any minerals, dissolved in it.  We use pure water to clean solar panels, and occasionally to clean windows as well.  When using pure water to clean, it dries spot free.  However, if there is heavy rain with strong winds, sometimes your windows will be affected by the rain.  That is why we offer a 7 Day Rain Guarantee.  If it rains within 7 days of your cleaning and your windows are affected, let us know and we will be happy to come out and touch up the exterior of your windows for free.