When Should Antibiotics Start Working?

I've had a sinus infection going on three months. I kept putting off going to the doctor figuring it would clear up. Yet I was in such misery I finally broke down and went last Tuesday. He put me on prednisone and an antibiotic. Okay, so today is Sunday. I'm a little bit better but not much...still VERY congested. Nothing seems to work! I seriously am so sick of feeling sick and miserable...I feel like I've been sick forever with no end in sight...that I just want to bawl. This is day five of the antibiotic with another five days to go...I'm on 875 mg twice a day which I do realize is a mive dose. When am I going to get better? And if antibiotics don't help, I don't know what other options I have, I've already had this a long time and I'm getting desperate.