2,088. A Perpetual Zavah
Hilchos Issurei Biah 7:15
If a woman’s pattern is to find blood on two impure days followed by two pure days, she may only engage in intimacy on the eighth, twelfth, sixteenth and twentieth nights.
Hilchos Issurei Biah 7:16
If her pattern is to see blood for three impure days followed by three pure days, she may engage in intimacy on two of the three pure days that follow her niddah days. The first of these must be watched corresponding to the two impure days that follow her niddah days, After that, she may never again engage in intimacy because she will be rendered a major zavah but she will never be able to count seven clean days.