How long does it take for a uti to show up on a urine culture, if you have an infection?

48 hours: Usually within 48 hours.

It may never show up: Utis is a clinical diagnosis - that means for a physician, we rely mainly on the patient's symptoms (pain on urination, hesitancy, frequency, flank/supra pubic pain) to make a dx. In many cases, we can grow and isolate the offending organism in the lab in <24hrs.. In other cases, it is much harder to do this (eg chlamydia, ureaplasma, mycoplasma) - sometimes resulting in a negative culture.