Hadith.4804 - Muhammad ibn Muslim asked Imam (as) about the waiting period of a mustahadah (a woman experiencing continuous bleeding).
Imam (as) said: "She should wait for the duration of her usual menstrual cycles, adding a day or subtracting a day. If she does not menstruate, then she should look at the menstrual cycles of some of the women like her and observe her waiting period according to their cycles."