How long does it take a urinary tract infection to reach the kidneys?

Variable: It depends on the nature of the bacteria, the persons immunity, the persons anatomy of the bladder and the ureters, and the persence of absence of stones. Of course, the trend of the symptoms of infection may predict if getting worse. Also, loin pain and fever will suggest kidney involvement.

Instantly ->few days: Bacteria can reach the kidneys almost instantly if the subject has vesico-ureteral reflux (urine backs up from bladder), or can take hours to days depending upon the bacteria. Some bacteria have tiny hair like projections (cilia) and can crawl through bladder vales and work their way up to the kidney. Sign of infection reaching the kidneys is when you develop back or flank pain.

UTI: This can happen within 1-2 days, several months, or not at all.