There are a variety of factors that can affect the duration of a bout of sinusitis, from the causative factor, to the sinus involved, and the treatment you use to address it.
Acute sinusitis, such as that caused by the common cold, typically should not last any longer than 4 weeks. Most often, those who experience the occasional sinus infection should notice their symptoms starting to wane by the 2nd week, given that they are observing the proper care and treatment.
If symptoms persist for 4 to 12 weeks, then you might have what doctors call a subacute sinus infection.
Individuals who experience 2 to 4 episodes of acute sinusitis in a year may have what doctors call recurrent acute sinusitis. This frequent, repeated incidence of sinus infection may be caused by seasonal allergies or anatomical anomalies concerning the structures near or involving the sinus.
Then finally, there is chronic sinusitis, which is characterized by sinus infections lasting beyond 12 weeks. Usually, doctors pinpoint allergies as the most likely causative factor for chronic sinusitis; however, anatomical issues may also play a role.