On average, how long will a bladder infection last?

Bladder infection. As long as want it to last if you don't treat it.
Short w antibiotics. Bladder infections should be treated with antibiotics. Simple infections usually repond to antibiotics within 24 - 48 hrs if on the correct antibiotic, and are usually cleared within 3 days. Always best to have urine culture performed to insure the infective germs are sensitive. U could well be on an inappropriate antibiotic if cystitis fails to clear up rapidly. Drinking lots of water helps.