When does the ruling of establishing the menses as six or seven days apply if continuous bleeding persists into the second or fourth month?

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The explicit ruling dictates that she is only reverted to six or seven days beginning in the fourth month. This is because the bleeding has not exceeded that established time if she were not deemed to be in a state of Istihadah. Therefore, the ruling is more applicable if she is in Istihadah. Al-Qadi suggests that the transition to six or seven days might take effect in the second month without needing repetition, given that her state of Istihadah is already confirmed, negating the need for repetition in her case.