Praise be to Allah.
If a woman’s menstrual cycle is irregular, then she should distinguish between the kinds of bleeding. When she sees menstrual blood, which is “black” blood (i.e., very dark red), recognizable and has a distinctive odour, then she should stop praying etc. until she becomes pure.
Shaykh ‘Abd al-Kareem al-Khudayr.
This is so long as it does not exceed fifteen days per month.