Although stains are rabbinically ordained, a woman who finds a stain of forbidding size in a location where concern is warranted must count seven clean days; the count begins the day after the blood ceases completely, with the timing and procedure varying based on whether she experienced bleeding for one day or multiple days.
וְנִמְצָא שֶׁכָּל הַנָּשִׁים הַיּוֹם, אֲפִלּוּ בִּרְאִיָּה אַחַת דִּינָן כְּזָבוֹת גְּדוֹלוֹת וּצְרִיכוֹת לִסְפֹּר שִׁבְעָה יָמִים נְקִיִּים, וְכֵן כָּל אִשָּׁה שֶׁמָּצְאָה כֶּתֶם בְּמָקוֹם שֶׁרָאוּי לָחוּשׁ לוֹ, אַף עַל פִּי שֶׁהַכְּתָמִים מִדְּרַבָּנָן (שם נח ב), כִּי מִן הַתּוֹרָה אֵין אִשָּׁה מִתְטַמְּאָה עַד שֶׁתַּרְגִּישׁ בְּעַצְמָהּ דַּם טֻמְאָתָהּ, שֶׁנֶּאֱמַר (ויקרא טו יט) דָּם יִהְיֶה זֹבָהּ בִּבְשָׂרָהּ. כְּלוֹמַר, שֶׁתַּרְגִּישׁ אוֹתוֹ, אַף עַל פִּי כֵן כְּבָר הֶחְמִירוּ חֲכָמִים עַל הַכְּתָמִים וְהִצְרִיכוּ סְפִירַת שִׁבְעָה עֲלֵיהֶם וְהֶפְסֵק טָהֳרָה. וּלְפִיכָךְ אִם מָצְאָה כֶּתֶם כַּשִּׁעוּר הָאוֹסֵר בְּמָקוֹם שֶׁרָאוּי לָחוּשׁ בּוֹ, צְרִיכָה לֵשֵׁב עָלָיו גַּם כֵּן שִׁבְעָה נְקִיִּים. וּמֵאֵימָתַי הָאִשָּׁה מוֹנָה שִׁבְעָה יָמִים נְקִיִּים? מִמָּחֳרַת יוֹם שֶׁפָּסַק הַדָּם לְגַמְרֵי. כֵּיצַד, רָאֲתָה שְׁנֵי יָמִים אוֹ שְׁלֹשָׁה בּוֹדֶקֶת עַצְמָהּ תָּדִיר, אִם פָּסַק הַדָּם בְּיוֹם שֵׁנִי אוֹ בְּיוֹם שְׁלִישִׁי אֲפִילּוּ בְּשַׁחֲרִית, אֵינָהּ מוֹנָה שִׁבְעָה מֵאוֹתוֹ יוֹם שֶׁפָּסַק הַדָּם בּוֹ, אֶלָּא לְמָחָר בּוֹדֶקֶת עַצְמָהּ פַּעַם אַחֶרֶת וּמַתְחֶלֶת לִסְפֹּר שִׁבְעָה מֵאוֹתוֹ הַיּוֹם. וּבַמֶּה דְּבָרִים אֲמוּרִים? שֶׁרָאֲתָה שְׁנֵי יָמִים אוֹ שְׁלֹשָׁה אוֹ עוֹד, אֲבָל אִם לֹא רָאֲתָה אֶלָּא יוֹם אֶחָד אַף עַל פִּי שֶׁבְּאוֹתוֹ הַיּוֹם בָּדְקָה עַצְמָהּ וְהִפְסִיקָה בְּטָהֳרָה אֵינָהּ מוֹנָה מִיּוֹם הַמָּחֳרָת, שֶׁחֶזְקַת הַיּוֹם הָרִאשׁוֹן כֻּלּוֹ טָמֵא, וְהֻחְזַק מַעְיָן פָּתוּחַ כָּל אוֹתוֹ הַיּוֹם, וְחוֹשְׁשִׁין שֶׁמָּא דָּמֶיהָ חָזְרוּ לָהּ לְאַחַר בְּדִיקָתָהּ וְשָׁפְעָה כָּל הַלַּיְלָה, וְאֵין יוֹם שֵׁנִי זֶה לִרְאִיָּתָהּ מִן הַמִּנְיָן, וַאֲפִילּוּ בָּדְקָה עַצְמָהּ מִבָּעֶרֶב חוֹשְׁשִׁין שֶׁמָּא עִם סִלּוּק יָדֶיהָ רָאֲתָה, אֶלָּא אִם כֵּן הָיוּ יָדֶיהָ בֵּין עֵינֶיהָ כָּל בֵּין הַשְּׁמָשׁוֹת (עי' רמב"ם איסורי ביאה פ"ו ה"כ), כְּלוֹמַר, שֶׁבָּדְקָה עַצְמָהּ וְהִנִּיחָהּ הָעֵד בִּפְנִים מִבְּעוֹד יוֹם וְעָמַד שָׁם בֵּין הַשְּׁמָשׁוֹת עַד הַלַּיְלָה וְלֹא מָצְאָה עָלָיו דָּם כְּלָל, בְּעִנְיָן זֶה אֶפְשָׁר שֶׁנֹּאמַר שֶׁתַּתְחִיל לִמְנוֹת מִיּוֹם הַמָּחֳרָת. וְדִינֵי שִׁעוּרֵי הַכֶּתֶם (שם נח, ב) וּבְאֵיזֶה עִנְיָן תּוֹלָה בַּכֶּתֶם אוֹ אֵינָהּ תּוֹלָה, וּמִי שֶׁיֵּשׁ לָהּ מַכָּה בְּאוֹתוֹ מָקוֹם אִם תּוֹלָה בָּהּ דָּמֶיהָ עַד שֶׁתִּחְיֶה הַמַּכָּה, וְעִנְיַן קְטַנָּה שֶׁנִּבְעֲלָה וְהַדָּם שׁוֹתֵת מִמֶּנָּה מָה דִּינָהּ (שם סד ב), וְדַם בְּתוּלִים שֶׁדָּנוּ אוֹתוֹ חֲכָמִים כְּדַם נִדָּה (שם סה ב), וְדִינֵי הַוְּסָתוֹת רַבִּים, וְדִין פְּרִישָׁה מִן הָאִשָּׁה סָמוּךְ לְוִסְתָּהּ שֶׁהִיא הָעוֹנָה, כְּלוֹמַר, אוֹתוֹ הַיּוֹם אוֹ אוֹתוֹ הַלַּיְלָה שֶׁהִיא רְגִילָה לִרְאוֹת בָּהֶן אֲסוּרָה בְּתַשְׁמִישׁ, מִדִּכְתִיב (שם לח) וְהִזַּרְתֶּם אֶת בְּנֵי יִשְׂרָאֵל מִטֻּמְאָתָם, וּבָא הַפֵּרוּשׁ, שֶׁיִּפְרְשׁוּ מִנְּשׁוֹתֵיהֶן סָמוּךְ לְוִסְתָּן, אֲבָל אַחַר שֶׁעָבְרָה עוֹנַת הַוֶּסֶת מֻתָּרִין לְשַׁמֵּשׁ. וְעִנְיַן הַהַרְחָקָה מִן הָאִשָּׁה שֶׁצִּוּוּ זִכְרוֹנָם לִבְרָכָה (שבת יא, א) בְּעִנְיַן הַנִּדּוּת כְּדֵי שֶׁלֹּא יִכָּשֵׁל אָדָם בַּעֲבֵרָה, וְהוּא שֶׁלֹּא יֹאכַל אָדָם עִם אִשְׁתּוֹ נִדָּה בִּקְעָרָה אַחַת, וְלֹא יוֹשִׁיט דָּבָר מִיָּדוֹ לְיָדָהּ (שם יג א), וְלֹא יְדַבֵּר עִמָּהּ דְּבָרִים שֶׁל הֶרְגֵּל עֲבֵרָה, וְכַיּוֹצֵא בִּדְבָרִים אֵלּוּ.
And it comes out that all women today are like a big zavah and need to count seven clean days, even with one appearance; and likewise any woman that found a [blood] stain in a place that it is fitting to be concerned about it. Even though stains are rabbinic (Niddah 58b) — as from Torah writ, a woman does not become impure until she feels impure blood herself, as it is stated (Leviticus 15:19), “blood flowing in her flesh,” meaning that she feels the impure blood in herself — nonetheless, the Sages were already stringent about stains and required the counting of seven [clean days] and a break of purity (hefsek taharah) for them. And therefore if she found a stain of the size that forbids in a place that it is fitting to be concerned about it, she also needs to sit seven clean days for it. And from when does a woman count her seven clean days? From the morrow of the day that the blood stopped completely. How is this? If she [experienced blood] for two or three days, she checks herself constantly. If the blood stopped on the second day or on the third day — even in the morning — she does not count her seven from that day in which the blood stopped, but rather she checks herself another time on the morrow and begins to count seven from that day. And what are these words speaking about? When she [experienced blood] two days or three or more. But if she only [experienced it] one day — even though she checked herself on that day and she had a break of purity, she does not count from the next day. As there is an assumption that the first day will be completely impure and the source is assumed to be open that whole day, and [so,] we are concerned lest her blood came back to her after her checking, and she was gushing the whole night. And [so] this second day from her [experiencing blood] is not from the number [of clean days]. And even if she checked herself from the evening, we are concerned lest with the removal of her hands [to check], she [experienced blood. This is] except if her hands were between her “eyes” all of the twilight (bein hashmashot) — meaning to say that she checked herself and left the [checking cloth] inside while it was still day and it stayed there all of the twilight until the night - and she did not find blood on it at all. In such a way, it is possible that we should say that she starts counting from the next day. (See Mishneh Torah, Laws of Forbidden Intercourse 6:20.) And the laws of the measure of the stain (Niddah 58b), and in which case it is attributed to a stain [from something else] and in which case it is not attributed [to it]. And one who has a wound in that place, if she attributes the blood to it until the wound heals. And the matter of what is the law of a girl minor who has had intercourse and has blood flow from her (Niddah 64b). And the blood of virginity (breaking the hymen), that the Sages determined to be like menstrual blood (Niddah 65b). And the many laws of the regular times, and the law of separation before her regular time, which is the [menstrual] period — meaning to say that day or that night that she is accustomed to [experiencing blood] upon it — is forbidden for sexual relations. [This is] from that which it is written (Leviticus 15:31), “And you shall separate the Children of Israel from their impurity,” and the explanation came that they should separate from their wives before their regular times. But after the period of the regular time has passed [without blood], they are permitted in sexual relations. And the matter of the distancing from the wife that they, may their memory be blessed, commanded (Shabbat 11a) regarding the matter of menstruation, in order that a man not stumble in sin — and it is that a man should not eat with his menstruant wife from the same tray, nor pass a thing from his hand to her hand (Shabbat 13a), nor speak to her about things that often cause sin nor similar to these things.