وَإِذَا زَادَ الدَّمُ عَلَى الْعَشَرَةِ، وَلِلْمَرْأَةِ عَادَةٌ مَعْرُوفَةٌ: رُدَّتْ إِلَى أَيَّامِ عَادَتِهَا، وَمَا زَادَ عَلَى ذَلِكَ فَهُوَ اسْتِحَاضَةٌ.
Meaning: If bleeding passes ten days and the woman has an established menstrual habit, she returns to the days of that habit, and the excess is istihadah.
An established habit is the previously recognized menstrual duration. Once bleeding exceeds ten days, this habit becomes the criterion for dividing hayd from istihadah.