The course of treatment is usually 14 days.
If you have persistence of symptoms beyond 10-14 days, you should be considered for second-line antibiotic therapy (more expensive antibiotics with wider spectrum)

Do you have sinusitis for the first time?
For how long you have symptoms?

Yes the course of antibiotic treatment is usually 10-14 days. Some ENT (Ear/Nose/throat) specialists recommend 30 day treatment of antibiotics especially if it's recurrent.

The pain/pressure is due to inflammation. Are you using an antiinflammatory nasal spray? Nasonex or Flonase are good ones and need a prescription. You can use them daily to help reduce inflammation during an infection along with the antibiotic. You can also use the spray to prevent recurrent infections. It is safe and non addictive.

Another defense is keeping the mucosa moist. Using saline sprays are natural and can be used daily. Nasal irrigations are also helpful- you may have seen the Netti-pot used on Oprah. This can be used during a flare-up or as a preventative measure.

For pain relief, steaming by filling the sink up with hot water and placing a towel over your head and sealing the area around the sink- breathe in deeply to inhale the steam. Hot showers and warm moist cloths on your affected sinuses will also help.

If it doesn't clear up after using many or all of these suggestions (give it 3 weeks), I would ask for a CT scan to rule out other problems.