Some of the things that change how long an air freshener scent lingers are the size of the air freshener, the humidity levels, wind or air flow, sunlight and temperature. Initially, the scents will be quite strong but will dissipate over several days and weeks, you will also get used to the smell so it may seem like it fades. We like to say that they last 1-2 weeks.
We always suggest cutting a small hole in the exterior bag of the air freshener or even a small slit in it and let the scents escape slowly. This can help the scent last up to 4 weeks.
An unopened air freshener will retain the scent up to 2 years if properly stored (not in the heat or sunlight).