I say, and with God's success: If it is less than ten days, then if the remaining time is sufficient for the washing and the prayer's opening takbir, the prayer becomes obligatory; if it is not sufficient, it does not become obligatory. The time for washing is counted here from the duration of menstruation; and it should be considered what is sufficient for washing from menstruation and the beginning of the prayer's opening takbir; because the cessation for less than ten days allows for the possibility of menstruation returning due to the remaining duration. Refer to: Sharh Al-Wiqayah p.124, Umdat Al-Ri'ayah 1: 128, and Dhakhr Al-Mutahilin p.135, and God knows best.