This applies to nasal decongestants (NOT nasal steroids). Nasal decongestants (such as oxymetazoline AKA afrin, or phenylephrine based medications) are vasoconstrictors. They work very well and work very quickly as the vasoconstriction (constricting the blood vessels) which shrinks the inferior turbinates (and any other edematous tissue).

The body responds to chronic vasoconstriction by making more blood vessels. When the nasal tissues have more blood vessels (and I presume are more dense with vessels) it's harder for the decongestant to work. This is called rebound congestion

conversely, the patients in this scenario will feel they need to use more decongestant since it previously worked so well, but it no longer does. This cycle can be challenging to treat.

For this reason most ENTs, including myself, typically recommend against afrin use for more than 3 consecutive days. I've seen who go as long as five, but I'm cautious and would not recommend more than 3 days.

It's a bit funny, because if you come into my clinic and get an endosocpic exam of the nose and/or throat (i.e. probably around 50%, often more, of my patients on any given day), I will spray afrin and lidocaine into the nose before my examination. The other main thing I use it for is nosebleeds. It's okay to use it for 3 days during an acute exacerbation of sinusitis, but I don't really think it's necessary.

Edit: I forgot to mention nasal steroids. As I said, the above response doesn't apply to them. We don't include nasal steroids in this because they have a very slow effect and don't have the effect of rebound congestion. With few exceptions doing 2 sprays each nostril daily for a very long is fine for almost everybody, and usually helpful. When I prescribe them I recommend patients use them for at least 4 weeks. Once in awhile there are patient that I would be more cautious with prescribing nasal steroids, such as those with a septal perforation, or frequent nose bleeds. Usually it's a non issue. Tip: when spraying them don't spray straight back -- use your opposite hand and spray towards the eye (i.e. spray with right hand into left nostril, aiming towards left eye).