Mishnah Niddah 9:10: If she is accustomed to regularly seeing [blood] on the fifteenth day [of the month], and she changed to seeing on the twentieth day, this [day] and that are forbidden [for her to have relations with her husband]. If she changed to [seeing on] the twentieth day twice, this [day] and that are forbidden. If she changed to [seeing on] the twentieth day three times, the fifteenth becomes permitted, and she has established the twentieth for herself [as the time of her regular cycle], for a woman can only establish a regular cycle for herself if she establishes it three times. And she can only be purified from [becoming impure on] her regular cycle once she is uprooted from it [i.e. it does not occur as expected] three times.

הָיְתָה לְמוּדָה לִהְיוֹת רוֹאָה יוֹם חֲמִשָּׁה עָשָׂר וְשִׁנְּתָה לִהְיוֹת רוֹאָה לְיוֹם עֶשְׂרִים, זֶה וָזֶה אֲסוּרִין. שִׁנְּתָה פַעֲמַיִם לְיוֹם עֶשְׂרִים, זֶה וָזֶה אֲסוּרִין. שִׁנְּתָה שָׁלשׁ פְּעָמִים לְיוֹם עֶשְׂרִים, הֻתַּר חֲמִשָּׁה עָשָׂר וְקָבְעָה לָהּ יוֹם עֶשְׂרִים, שֶׁאֵין אִשָּׁה קוֹבַעַת לָהּ וֶסֶת, עַד שֶׁתִּקְבָּעֶנָּה שָׁלשׁ פְּעָמִים. וְאֵינָהּ מִטַּהֶרֶת הַוֶּסֶת, עַד שֶׁתֵּעָקֵר מִמֶּנָּה שָׁלשׁ פְּעָמִים:

Mishnah Niddah 9:11: Women, with regard to their virginal blood, are like grapevines. There are vines whose wine is red, and there are vines whose wine is black, and there are vines whose wine is abundant, and there are vines whose wine is meager. Rabbi Yehuda says: every vine has wine in it; and that which does not have wine in it is dried up [i.e. infertile].

נָשִׁים בִּבְתוּלֵיהֶם כַּגְּפָנִים. יֵשׁ גֶּפֶן שֶׁיֵּינָהּ אָדֹם, וְיֵשׁ גֶּפֶן שֶׁיֵּינָהּ שָׁחֹר, וְיֵשׁ גֶּפֶן שֶׁיֵּינָהּ מְרֻבֶּה, וְיֵשׁ גֶּפֶן שֶׁיֵּינָהּ מֻעָט. רַבִּי יְהוּדָה אוֹמֵר, כָּל גֶּפֶן יֶשׁ בָּהּ יַיִן. וְשֶׁאֵין בָּהּ יַיִן, הֲרֵי זֶה דּוֹרְקְטִי: