How long before treating a uti before symptoms beome serious?

Depends on factors: Once an takes hold in the urinary tract, it can move upward and infect the kidneys. The rate that this happens depends on individual vulnerabilities such as anatomy, age, immune function, past history of infection, causative agent, treatment choice. Signs that an infection is serious: fever, chills, vomiting, diarrhea, unable to eat, pain in flank (where kidneys are located), malaise.