Generally, 48 hours. If someone requires catheter for prolonged period, with excellent care, it can be changed every 3-4 weeks. But this require excellent skilled nursing care under guidance of a urologist.
Immediately. Placing a catheter in the urinary tract carries a small risk of infection. If the catheter remains in place, there is 100% colonization by bacteria in 2 weeks. This colonization should not be treated as this will only lead to antibiotic resistance. With a chronic foley catheter, antibiotics should only be given if a symptomatic infection arises.
Almost right away. Each time a catheter is put into a bladder, the risk of infection is about 1%, even with the best technique and careful nurse/doctor. When a catheter is left in the bladder, the risk of infection goes up and can be as high as 10% per day. That's high, but there are many times when it's necessary. If you or someone you know has one, have the patient ask the doctor to take it out as soon as possible.