Is it necessary to finish antibiotx that didn't help treat infctn b4 changing it?daughter is in 5th day, no improvements gp wants to finish b4 a new one

Confusing question: Depending on the type of infection I am treating, i may want to give a antibiotic a few extra days to see if it will eventually work.Once infection is established, it does take time to build up a kill level of antibiotic in the body to attack the germ. If it is obvious it is not working, i stop it and try something different.