Taking keflex for sinusitis since tuesday (have taken 5 doses). still have sinus pain and discharge. it is too soon to tell if it's working?

Maybe too soon: Sinus infections usually require longer treatment times with antibiotics so it is not unusual for it to take longer before the antibiotic starts to take effect. See your doctor if you are experiencing worsening symptoms, or don't notice substantial improvement as you near the end of the course of antibiotics.

24-36 hours: I usually tell my patients it will be 24-36 hours before they feel some relief.. So you are a little outside that window. I personally do not use Keflex to treat sinusitis. I also tend to use a steroid dose pack as well. You may want to try some over the counter afrin (oxymetazoline) to decongest your nasal/sinus passgages to help relieve the pressure.