Air freshener refills should generally be replaced every 30 to 60 days, depending on several factors such as the type of air freshener, the size of the space, usage frequency, and personal preference for scent intensity. Automatic spray air fresheners often have settings that allow you to control the frequency of sprays, which can affect how long a refill lasts. For example, if set to spray every 9, 18, or 36 minutes, the refill duration will vary accordingly. 
Gel-based air fresheners typically last around 30 days, while plug-in air fresheners can last between 30 to 60 days, depending on the brand and settings. Reed diffusers may last longer, often up to 90 days, but the scent may diminish over time. 
In high-traffic areas or larger spaces, refills may need to be replaced more frequently to maintain the desired scent level. Conversely, in smaller or less frequently used areas, refills might last longer. Environmental factors such as humidity, temperature, and ventilation can also impact the longevity of air freshener refills. 
Ultimately, the replacement frequency can be adjusted based on personal preference for scent strength and the specific conditions of the space. Regularly checking the air freshener and monitoring the scent level can help determine the optimal time for replacement.