Firstly: The Greatest Duration of Purity
There is no upper limit for the duration of purity. Consensus has been quoted on this matter by: al-Kasani, al-Nawawi, Ibn Taymiyyah, and al-Shinqiti.
Secondly: The Shortest Duration of Purity
There is no lower limit for the duration of purity. This is the position of the Zahiris, an opinion of al-Shafi`i, a narration from Ahmad confirmed by al-Mirdawi, the position of Ishaq, and the choice of Ibn Taymiyyah, Ibn Baz, and Ibn `Uthaymin.
Thirdly: The Signs of Purity from Menstruation White strip  A thin, thread-like discharge that is ejected by the uterus once the cycle is concluded.
If the menstruating woman sees white discharge in the form of strip or thread, then she is pure. This is by agreement of the four schools of jurisprudence. Dryness
If a menstruating woman wipes her private part with cloth and it returns dry, then she has finished her cycle and is pure. This is by agreement of the four schools of jurisprudence.
Fourthly: Washing After Purity from Menstruation  This has been discussed in the reasons for ritual washing, subtitle: “11. Ending of menstruation or post-natal bleeding”