On average, how long does it take for an oral antibiotic for a skin infection take to take effect?

Hard to say!: There are lots of different germs that can cause infections. No antibiotic helps them all. Doctors make an educated guess of which antibiotic has a good chance of working,(depending on location, health of person & symptoms of the infection), if they don't take a sample for culture, That drug may be effective, partially effective or ineffective, only time will tell. F/u if no change at all in 2-3 d