The filters should be changed every 3 months to ensure optimum extraction performance, although this may vary depending on what kind of cooking you normally do, and how often. If you have a SMEG hood with electronic controls, an indicator light will tell you when it's time to change the filters. Clean filters are essential for optimum performance.
Never clean the appliance using abrasive cloths or aggressive detergents such as bleach, ammonia or alcohol, as this could leave permanent marks on the surface. Use only soft, clean cloths, and non-abrasive cleaning fluids, specifically meant for the surface that you are cleaning, like stainless steel cleaning fluid.