The answer to this question can vary quite a bit depending on weather, location and your personal preference.
Here in Southern Illinois humidity is an ongoing issue. High humidity fosters mold and mildew growth. Nearby water sources such as lakes, ponds and even low lying areas in your yard with poor drainage cause extra humidity to surround your home.
You will also notice that the north side of your home will tend to grow mold and mildew quicker than the other sides. This is because the north side gets the least amount of sunshine and therefore stays wet longer. Without sunshine to dry out the siding and the ground around it, mold and mildew will prosper.
Washing Mildew on a log cabin
Lastly, it comes down to how clean you want to keep your home. Some people may not care about the green color of their white siding. They may let it go for years without having it washed. But what you must consider is that mold tends to spread and it will spread to areas that might not be visible to the naked eye. It could spread underneath your vinyl siding to cause serious and costly damage. If the growth is on painted surfaces then the mold will slowly break down the paint and it will be needed to be painted again sooner. This will cost a lot more than having it washed!
Most of our customers have their houses washed every two years but some may need it sooner. If you wait too long you could end up costing yourself thousands of dollars in costly mold removal or painting repairs in the future.