Unless they are damaged, permanent filters don’t need to be replaced, but they must be cleaned every three months or so, depending on usage. The life cycle of replacement filters, on the other hand, varies. Most activated charcoal filters should be changed after 3,000 hours - or 125 days - of use, but if you cook and fry often, you might need to replace them after three or four months. When used in homes, HEPA filters can last up to two years, but may need to be changed twice a year. It’s best to follow what your instructions recommend. For those who have trouble keeping track of time, some devices will warn you when it’s time to replace a filter.