Depending on your environment, ( lot of dust or little amount of dust) would be the need for changing the filter. Our air purifier has 2000 hours replacement alert, when the air filter is used for more than 2000 hours, the indicator at the top will turn red to remind you that the air filter needs to be replaced. After replacing the filter, long press the "Light" button for 2 seconds, and the red indicator light will flicker 3 times with 2"beeps" when it is re-set, it will re-count the lifespan of the filter.