All perfect praise be to Allah, The Lord of the Worlds, and peace and blessing be upon the Messenger of Allah, his family, and all his companions.
Do not pray until the menstruation ends completely, either by seeing the white discharge, which is a white substance that comes out at the end of the menstruation when it stops or by dryness, which is when the woman inserts a piece of cotton or a piece of clothing or something alike into her vagina and it comes out without any trace of blood, or yellowish or brownish discharges.
These are the two signs of purification; if the woman sees either of them, she makes Ghusl and prays. Sharh Kifayat At-Talib Ar-Rabanni reads about the white discharge and dryness: “Qassah” is a white discharge that comes at the end of the menstruation period, and it is a sign of womb’s clearance, as for the sign of dryness, is it when the woman enters a piece of clothing or cotton into her vagina, and then it comes out dry without wetness.”
Al-Hattaab said in (Mawaahib Al-Jaleel): “He did not mean to be completely free from wetness, rather, it means to be free from blood, yellowish or brownish discharges because the woman’s vagina usually is not free from wetness.”
Allah knows best.