I say, and with God's success: If her purity lasts for ten [days], the prayer becomes obligatory, and if the remaining time is just a moment, even as long as it takes to say: Allah; because the cessation of blood for ten [days] is a certain purity, as the menstrual period does not exceed this duration. Anything beyond that is considered istihada (irregular bleeding). Refer to: Sharh al-Wiqayah p. 124, Umdat al-Ri'ayah 1: 128, and Dhakhr al-Mutahhilin p. 135, and God knows best.