A 24-year-old woman presents with a 1-week history of dysuria, increased urinary frequency, and suprapubic discomfort. She is otherwise well and has no vaginal discharge or itch. Urine dipstick is positive for leucocytes and nitrites. She has no known drug allergies. According to NICE guidelines for uncomplicated lower UTI in non-pregnant women, what is the recommended first-line antibiotic treatment duration?