السلام عليكم و رحمة الله و بركاته
I would like to ask that when a woman is on haidh and her habit is of 7 days but the next month she comes on haidh for 9 days, will day 8 and 9 of this month be classed as istihadha, and therefore the month after this her habit will change to 9? Or those 9 days will be classed as haidh and there would be no istihadha as it is in the ten days?
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
In the name of Allah, the Most Gracious, the Most Merciful
If a woman’s bleeding exceeds her usual habit but remains within the boundary of the maximum number of days Haidh can be for (i.e. 10 days), then it will be considered that her Haid habit has changed. So, in the given scenario, days 8 and 9 will be part of her Haid and now her Haid habit will be 9 days.
Only Allah knows best
Written by Maulana Muddasser Dhedhy
Check and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
 الطهر المتخلل بين الدمين و الدماء في مدة الحيض يكون حيضا. (الفتاوى الهندية ج 1 ص 41. دار الكتب العلمية)
Birgivi's Manual Interpreted: Complete Fiqh of Menstruation & Related Issues pp. 39-42.