After a woman becomes a Nidda, she must count seven "clean days" after establishing the cessation of bleeding, and she may then immerse in the Mikveh upon the conclusion of the seven "clean days."
There is a debate among the Halachic authorities as to when a woman may begin counting the seven "clean days." Before presenting the different opinions, it must be emphasized that according to all views, the woman cannot begin counting the seven "clean days" until she’s established that she is indeed "clean," meaning, that all bleeding has ceased. It goes without saying that the seven "clean days" do not begin until the woman makes an inspection and verifies that the bleeding has stopped. Generally speaking, upon becoming a Nidda a woman bleeds for four, five or six days. The Halachic debate we present here relates to the question of how early a woman who confirmed the cessation of bleeding may begin the seven "clean days."
Maran, in Shulhan Aruch, writes that a woman may begin the seven "clean days" after four days since she became a Nidda. Meaning, the minimum number of days that will have to pass before immersion is eleven (4 + 7). The Rama (Rav Moshe Isserles of Cracow, 1525-1572), however, documents the custom to wait five days before beginning the seven "clean days." There is also a third view, that of the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), who says that the custom in Baghdad was to wait seven days before beginning the seven "clean days."
Hacham Ovadia Yosef ruled in accordance with Maran’s position, that a woman who ascertained the cessation of bleeding after just four days may then begin counting the seven "clean days." However, the accepted custom among Syrian Jews, which seems to have been the custom practiced in Syria, follows the custom recorded by the Rama, requiring five days before beginning the seven "clean days." It is likely that due to the particularly grave nature of the prohibition of Nidda, the Syrian-Jewish community accepted this practice as a matter of stringency. Regardless, this is the policy taught in Kallah classes in our community, and this was the position taken by Hacham Baruch Ben-Haim zs"l.
In deference to our community’s accepted custom, and especially in light of the fact that most women in any event do not completely stop bleeding before five days have passed, it is proper to follow this practice. Of course, if, for whatever reason, a woman immersed in a Mikveh after eleven days since the onset of bleeding, her immersion is nevertheless valid, assuming that she had verified the cessation of bleeding before counting seven "clean days."
It should also be noted that, as Hacham Ovadia cites (Taharat Ha’bayit, vol. 2, p. 412), those authorities who require waiting five days allow a woman to begin the seven "clean days" after just four days under certain extenuating circumstances. One example is a situation where the immersion would be scheduled for the night after the husband is leaving on a business trip – meaning, if the woman begins the seven "clean days" after five days, then her immersion would take place, let’s say, on Wednesday night, and the husband is traveling on Wednesday afternoon. In such a situation, if the woman is able to establish the cessation of bleeding after four days, she would be allowed to begin the seven "clean days" at that point, so she can go to the Mikveh and be permissible to her husband on Tuesday night, before his trip. It must be emphasized, however, that each case must be brought to a Rabbi for his consideration. One cannot assume that whenever immersing on a certain night poses inconvenience it is permissible to begin counting the seven "clean days" one day earlier. One should abide by our communal custom and wait five days unless a Rabbi has determined that the counting may begin earlier due to extenuating circumstances.
Summary: The custom among Syrian-Sephardic women is to begin counting the seven "clean days" five days after becoming a Nidda (assuming the woman has confirmed the cessation of bleeding). Under extenuating circumstances, she may begin counting the "clean days" four days after becoming a Nidda; a Rabbi should be consulted for guidance when such situations arise.