What is the ruling on blood that flows for more than 35 days, and what is the duration of menstruation (hayd) and irregular bleeding (istihadah), and how are the 15 days between two menstrual periods calculated?

If the duration of the blood flow exceeds fifteen days, then you are experiencing Istihada (prolonged non-menstrual vaginal bleeding).

You should consider the duration of your previous menstrual period, which is seven days of menstruation. After that, you perform ghusl (ritual bath), pray, and are considered pure until your next menstrual period.

During Istihada, you must take precautions such as binding with a cloth or similar, and perform wudu (ablution) for each prayer after its time has begun. You then pray both obligatory and supererogatory prayers with that wudu.

The minimum period between two menstruations is fifteen days, meaning that blood returning after a period of purity lasting this long is considered a new menstrual period.

You should act according to your previous menstrual habit: whatever coincides with its duration is considered menstruation, and anything beyond that is Istihada.