الحمد لله رب العالمين ، وصلى الله على سيدنا محمد وعلى آله وصحبه أجمعين، وبالله التوفيق

When a woman is muʿtādah (she experienced haid) and she experiences menstrual bleeding that continues for 18 days without any intermittent period of non-bleeding and ceases for more than 15 days;

أو معتادة بأن سبق لها حيض وطهر فترد إليهما قدرا ووقتا وتثبت بمرة في الأصح المنهاج: 1: 136

If there is no variation in the strength of blood, she should differentiate between menstruation and the bleeding by referring to the last normal haid cycle. For example, her last haid is 5 days followed by 25 days of purity, therefore, she should consider the first 5 days of bleeding to be haid and the next 25 days to be purity.

ويحكم للمعتادة المميزة بالتمييز لا العادة في الأصح المنهاج: 1: 136

If there is variation in the strength of blood, she should classify her bleeding as either menstruation or purity based on its strength. For example, a woman who sees 10 days of red blood followed by 8 days of yellow discharge will consider the days of strong bleeding (i.e., red blood) to be menstruation and the 8 days of weak bleeding (i.e. yellow discharge) to be purity.

And Allah Most High knows best.