It really depends on the type of infection and the antibiotic prescribed for said infection. For me, and I've said this before but I am a MRSA carrier. I have so many antibiotic allergies and intolerances that the only ones that really work are the strong IV antibiotics. Nothing else really works.
If you are talking for things like tonsillitis, or some other thing it is usually a 7 day course of antibiotics, for something that is antibiotic resistant it is usually IV antibiotics and then a course of strong antibiotics for myself it was 4 months. I had a very bad Staff infection and had been battling it for approx,3 months before the strong IV antibiotics. It can be a long drawn out afair.