Halakhah for Shabbat 128:14

To not have intercourse with a menstruant woman: To not have intercourse with a woman when she is menstruant, as it is stated (Leviticus 18:19), "And to a woman in menstrual impurity, you shall not approach." And the time of her menstruation continues for seven days, as it is written (Leviticus 15:19), "seven days shall she be in her menstruation." And whether she [experiences blood] once during them or gushes all seven, she is a menstruant (Niddah 73a). And the entire time that she does not immerse (in a mikveh) - even after the seven - she is [considered] a menstruant; since the verse made it dependent upon days and immersion, as it is stated with impure people (Leviticus 15:18), "and they shall wash in water." And they, may their memory be blessed, said [that it is] a constructive paradigm (binyan av) to any impure one, that he is with his impurity until he immerses (see Rabbenu Bachya on Leviticus 15:19 in the name of the Geonim). And likewise, they, may their memory be blessed, also expounded (Shabbat 64b), "'Shall she be in her menstruation' - she shall be in her menstruation until she goes into water."