What is the ruling for a woman who has no established habit (La-'Ada) and no differentiation criteria (La-Tamyez)?

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This woman falls under the fourth category of those experiencing Istihada, which includes the forgetful woman (an-Nasiyah). The forgetful woman is divided into three types: one who forgets both the time and the number; one who forgets the number but remembers the time; and one who remembers the number but forgets the time. For the one who forgets both time and number, Al-Kharqi established the ruling that she observes six or seven days each month as her menses, after which she performs Ghusl and treats the subsequent days as Istihada, performing fasting, praying, and circumambulating (Tawaf). A narration from Ahmad states she sits for the minimum duration of menses. If she knows her customary month, she observes that duration in that month; if not, she observes it in the commonly known month as it is the prevailing state. Evidence for this is the Hadith of Humnah bint Jahsh, where the Prophet (peace be upon him) instructed her to calculate six or seven days as her menses, then perform Ghusl and pray and fast during the remaining days.