How long does it take for a uti (urinary tract infection) to show positive on a urine culture?

Immediately: Uti is caused by bacteria multiplying in the urine. Thus bladder urine will be full of bacteria once you have a uti.

Urine culture: As soon as 24 hours.

It may not!: A urinary tract infection is a clinical diagnosis, and doesn't necessarily hinge on a positive urine culture. However, typical bacteria associated with uti's will test positive within 24hr. Cultures help in choosing the most appropriate antibiotic.However, keep in mind some microbes cannot be cultured, or aren't routinely cultured (e.g. Ureaplasma, chlamydia) - but can be treated with antibiotics.

Less than 24 hours: It usually takes less than 24 hours to find out whether or not enough germs in urine. Identifying growing germs and determining appropriate antibiotics against them takes another 24 hours.