No more than a few hours. Granted, I was only on birth control a month and so my experience is limited ... but it came out with such speed that I would either have to wear a pad, or it looked like I had incontinence problems.
She might have an infection. White, clumpy discharge is a symptom of yeast infection. Ask her about other symptoms; tell her to go to the doctor ... something. I hope she's not been cheating on you.