How long does it take a kidney infection to clear up?

A few days: If someone with an uncomplicated bacterial UTI is given an antibiotic to which that the organism is sensisitve, it should clear up in a few days. Most antibiotic treatments are given 5-7 days. However, there are many reasons why it may not clear rapidly. Just a few are stones, poor bladder empyting, more aggressive organisms, bad kidney function, prior surgery on the urinary tract, etc.