Out of the many appliances with filters, vacuum filters have the most visible effect when it isn’t changed. When the vacuum filter is no longer functional, the vacuum will leave behind dust no matter how many times you empty the canister or bag. When that happens, it’s a good sign the filter needs to be cleaned or replaced. Check your vacuum filter every six months, at minimum, if you use it fairly regularly. If the filter is soaked and can’t be cleaned, it’s time to buy a new one. Otherwise, you can replace filters once a year to be safe.