A urinary tract infection (UTI) can take on average between a few days to a week to turn into a kidney infection if left untreated. The bacteria from the bladder infection can ascend through the urinary tract to the kidneys during this time, causing a more serious infection known as pyelonephritis. The exact time frame can vary based on individual factors such as overall health, immune system strength, age, and the type of bacteria involved. If a person is prone to infections or has a weakened immune system, the progression to a kidney infection can happen more rapidly. Recognizing and treating symptoms early—such as burning during urination, frequent urgency, pelvic pain, and cloudy urine—can prevent this escalation. Kidney infections bring more severe symptoms including fever, back pain, and nausea, and require prompt medical attention. Thus, if UTIs are untreated for several days, typically up to a week, they can develop into kidney infections, which are more serious and need antibiotic treatment to avoid complications like kidney damage or sepsis.