How long does it take for bactrum to start working for a sinus infection. Six days now of taking it. Alot of pressure in face and head still?

Perhaps not infectio. Perhaps it's not an infection. Other things can cause sinus symptoms, such as allergies. Most sinus infections by the way do clear up within 5-7 days without antibiotics. Infact lately there been alot of attention towards medically unnecessary treatments, and antibiotics for sinus infections is often one of them. Feel better!
Antibiotic alone? Sinus infections will seldom clear with antibiotics alone. Promoting drainage, relieving any obstruction to drainage, irrigation and rinses all may be necessary for successful management. If it persists despite these, suggest see ENT doctor.