It is imperative to clean the filters regularly otherwise there could be a risk of fire.
Sponge, wash with hot soapy water once a month and replace every 5 to 6 washes.
Metallic: Wash monthly with non-abrasive detergent, by hand or in a dishwasher on low temperature cycle.
Carbon filters are not washable and should be replaced every 6 months.
Paper: Replace once a month or if discoloured.