Most extractor hoods have stainless steel or aluminium filters. Generally speaking they do not need to be replaced, they just need to be cleaned. They have a wire mesh appearance and that is what traps the grease. They normally slide or pull out. We clean these by placing them in the heated dip tank inside our vans to remove all the grease and then we spray them down with a water jet. The filters are dried and then put back in place.
Built-in / Pull-out extractors normally have paper filters, sometimes with a carbon filter located behind it. We replace the paper filters as part of our service. Our filter papers go a red colour when they are coated with grease, indicating when it is time to change the paper filter again.
To clean the extractor units themselves involves lots of degreasing paste as they get very sticky and greasy inside and out. Even when the extractor is not in use, steam still rises into the extractor area, meaning it is constantly absorbing residue that then dries and forms layers of grease and fat.