It is narrated from Mujahid, may Allah have mercy on him, that « ﴿حَتَّىيَطْهُرْنَ﴾ » means when the flow of blood stops, and «﴿فَإِذَاتَطَهَّرْنَ﴾» said: that is, when she performs ghusl (ritual bath).
Hadith Referenceسنن دارمي / من كتاب الطهارة / 1117