Purity: Transition from Slight to Medium Istiḥāḍah
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 397:
If the slight istiḥāḍah of a woman becomes medium after ṣubḥ prayers, she must perform ghusl for ẓuhr and ʿaṣr prayers. If it becomes medium after ẓuhr and ʿaṣr prayers, she must perform ghusl for maghrib and ʿishāʾ prayers.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)