Mishnayos Niddah Perek 9 Mishnah 10

If the woman was accustomed to see the flow of blood on the fifteenth day and she deviated from the norm to see the flow of blood on the twentieth day, then on both this day, the fifteenth, and that day, the twentieth, it is prohibited for her to engage in intercourse. If she deviated from the norm twice, then on both this day, the fifteenth, and that day, the twentieth, it is likewise prohibited for her to engage in intercourse. If she deviated from the norm to see the flow of blood on the twentieth day three times, it becomes permitted for her to engage in intercourse on the fifteenth, and she has established the twentieth day for herself as the day of her fixed menstrual cycle, as a woman establishes a fixed menstrual cycle only after she establishes it three times. And a woman is purified from the existing fixed menstrual cycle, in the sense that intercourse is permitted on that day, only when she has been displaced from that day 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: