How long will it take for Zpack to kick in for strep and sinus infection?

I was diagnosed with strep and a sinus infection last Monday. After taking amoxicillin for 4 days, I went back to the doctor because there was no improvement. They changed me to Zpack today and I took my first 2 pills around 2:00pm today. Generally how long does Zpack take to kick in? The doctor and pharmacist said different things so i'm confused. Thanks!