As long as you get plenty of rest, stay hydrated and get plenty of Vitamin C most sinus infections to go away on their own without medical treatment.
Most sinus infections last from 2 to 4 weeks if they're not very severe and go way on their own with home treatment.
If the sinus infection lasts much longer than 4 weeks then you should see a doctor for medical treatment because it could be more severe.
Your doctor may also need to prescribe antibiotics for treatment of the sinus infection.