What is the ruling on intermittent dryness for one who does not know the duration of her menses, and sometimes sees the white discharge and sometimes does not?

A woman knows her purity from menstruation by one of two signs: dryness (al-jufūf) or the white discharge (al-qaṣṣah al-bayḍāʾ). If she sees either of them, she has become pure and must perform ghusl (definition) (ritual bath). If blood reappears after that, she reverts to being in menstruation as long as it is within the possible period of menstruation. Dryness is considered purity even if it is intermittent, and the woman performs ghusl when she sees it, then stops praying if blood reappears. This purity that interrupts menstruation is a valid purity. Some scholars permit waiting half a day or a full day to confirm purity during the accustomed period, but the fatwa (definition) given is that ghusl is obligatory upon seeing purity if she intends to do something that requires ghusl, such as prayer.