There are many different factors that will affect how often you should wash your home's exterior. The main one is climate, but you may also need to take into account what kind of paint you have on the surface, whether or not it's in a shady area, and whether there's a lot of traffic and pollution in your area.
Most experts recommend washing your home's exterior every 12-18 months. If you live in an area with harsh weather conditions and high traffic, it's important to wash it more often so that it doesn't become dull and discolored over time.