Thanks for your question.
The discharge that you noticed on her underwear is completely normal and occurs regularly for women, even if they are not having sex at all.
The vagina and cervix produce fluid regularly as a house-keeping mechanism to help cleanse the vagina and prevent infections. The amount and consistency changes throughout a woman’s menstrual cycle. It’s normal for a woman to regularly have some white discharge on her underwear. I am not able to say how long the discharge occurs because every woman is different and it changes throughout her lifetime.
I hope this has helped.