What is the ruling for a woman who has forgotten the specific month of her habitual menstruation but remembers the time frame?

Primary text

If a woman is ignorant of her habitual month, she must revert to the lunar month cycle, assigning one menstrual period (Hayd) in every lunar month. This ruling is based on the Hadith concerning Humnah and because the lunar cycle is the prevailing standard to which she should be returned, similar to how one reverts to a six or seven-day period.