You should probably be washing your windows at least twice a year or every 6 months in the spring and fall. This can help keep them clean and free of dirt, dust, and other contaminants that can build up over time. If you're located in an area with a lot of wind or rain, then you may want to wash your windows more often. Pollution from cars, industry, and other sources can also cause buildup on your windows, so if you live in an area with high levels of air pollution then you should consider washing them more frequently.