It usually takes three days or more until you start feeling better after you have started any antibiotic, it takes time for the medicine to work. I'd call the doctor if you still aren't feeling better - not totally well but better - by the time the pack is finished, and maybe they'll give you something else that's stronger, or ask you to come in again. The Z-pack is usually the first line of defense from many doctors, but I won't take it anymore, it doesn't help me. But there's always the chance you have a virus and it's not a sinus infection, so it may have to run its course. I hate when doctors say that, but it's true, and that's why they don't like to over prescribe antibiotics. Sorry you're feeling lousy, hope you feel better soon!