She should count the number of nights and days during which she used to menstruate each month before this condition came upon her, and she should refrain from prayer for that portion of the month. Once that period has passed, she should perform a full bath, then secure herself with a cloth, and then pray.
أخبرنا قتيبة عن مالك عن نافع عن سليمان بن يسار عن أم سلمة تعني أن امرأة كانت تهراق الدم على عهد رسول الله ﷺ فاستفتت لها
لتنظر عدد الليالي والأيام التي كانت تحيض من الشهر قبل أن يصيبها الذي أصابها فلتترك الصلاة قدر ذلك من الشهر فإذا خلفت ذلك فلتغتسل ثم لتستثفر ثم لتصلي
[Its chain is daif] Abu Dawud transmitted it in Kitab al-Tahara, chapter on the woman who experiences istihada, etc, hadith 274, from the hadith of Malik through this route, and it is found in Muwatta (Yahya): 1/62, and in Kubra, hadith 214. It contains a serious flaw, as the chain is disconnected. Part of the hadith has many corroborating narrations in Abu Dawud, hadith 281, and elsewhere.