Most Sinus Infections as well as bad Sinus Infections go away within 2 weeks without any treatment or antibiotics.
If the sinus infection does last longer than 2 weeks which is 14 days then seeing a doctor is a good idea because you might have a more severe sinus infection that could require antibiotics to treat the sinus infection.
When you have a Sinus Infection you should make sure to drink plenty of water, eat foods that have Vitamin C, use a neti pot to help clear out your nasal passages and use a humidifier or vaporizer in your room or home to add moisture to the air.
Then if the sinus infection still hasn't gone away within 14 days then go see a doctor or go to urgent care to get checked out.