Is it sufficient to insert the cotton once to ensure purity, or must one wait a certain period for no subsequent brown discharges, which would be considered menstruation, to appear?

A woman's purification from menstruation or post-natal bleeding (nifas) is recognized by one of two signs: the cessation of blood and the dryness of the place, such that a cotton swab comes out clean, or the emission of the white discharge (al-qassah al-bayda'). If the place becomes dry and the cotton swab comes out clean, this is a sign of purity, and the subsequent discharge of yellow or brownish secretions does not matter.