How long for antibiotics to take affect for respiratory infection?

I took in a cat that showed up at my house pregnant... she has since then had her kitties... I took her to the vet for a sneezing and wheezing problem and he said she has pneumonia and a respiratory infection... poor girl :( So how long does it normally take for a cat to start showing signs of becoming healthy again??? The vet gave her a shot and sent me home with antibiotics.