Mishnah Niddah 9:8: Any woman who has regular [indicative symptoms around her] cycles, her hour [of discovering menstrual blood] is sufficient for her [to reckon her period of impurity from that moment]. The following are considered regular [indicative symptoms around her] cycles: if she yawns [around the time that she menstruates], or if she sneezes, or if she has a [painful] feeling in her stomach or in her lower abdomen, or if she has a discharge, or if she is seized by a kind of chill [i.e. shivers or goosebumps], or other similar symptoms. Any woman who has has established three occurrences [of any one these symptoms], this is thereby a regular [indicative symptom for her] cycle.

כָּל אִשָּׁה שֶׁיֶּשׁ לָהּ וֶסֶת, דַּיָּהּ שְׁעָתָהּ. וְאֵלּוּ הֵן הַוְּסָתוֹת, מְפַהֶקֶת, וּמְעַטֶּשֶׁת, וְחוֹשֶׁשֶׁת בְּפִי כְרֵסָהּ, וּבְשִׁפּוּלֵי מֵעֶיהָ, וְשׁוֹפַעַת, וּכְמִין צְמַרְמוֹרוֹת אוֹחֲזִין אוֹתָהּ, וְכֵן כַּיּוֹצֵא בָהֶן. וְכֹל שֶׁקָּבְעָה לָהּ שָׁלשׁ פְּעָמִים, הֲרֵי זוֹ וָסֶת:

Mishnah Niddah 9:9: If she is accustomed to regularly seeing [blood] at the beginning of her regular [experiencing of indicative symptoms of her] cycle, all pure items which she handled during her regular [indicative symptoms of her] cycle are impure. If [she is accustomed to regularly seeing blood] at the end of her regular [experiencing of indicative symptoms of her] cycle, all pure items which she handled during her regular [indicative symptoms of her] cycle are pure. Rabbi Yose says: even days and hours can be [indications of] regular cycles [i.e. if she regularly sees blood on certain days or hours, she can establish a regular cycle based on that]. If [for instance] she is accustomed to regularly seeing [blood] at sunrise, she is not forbidden [to her husband as a niddah on the day of her regular cycle] until sunrise. Rabbi Yehuda says: the entire day is hers [i.e. if she did not see blood after sunrise, she is permitted to her husband for the entire day, but not the night before].

הָיְתָה לְמוּדָה לִהְיוֹת רוֹאָה בִתְחִלַּת הַוְּסָתוֹת, כָּל הַטָּהֳרוֹת שֶׁעָשְׂתָה בְתוֹךְ הַוְּסָתוֹת, טְמֵאוֹת. בְּסוֹף הַוְּסָתוֹת, כָּל הַטָּהֳרוֹת שֶׁעָשְׂתָה בְתוֹךְ הַוְּסָתוֹת, טְהוֹרוֹת. רַבִּי יוֹסֵי אוֹמֵר, אַף יָמִים וְשָׁעוֹת, וְסָתוֹת. הָיְתָה לְמוּדָה לִהְיוֹת רוֹאָה עִם הָנֵץ הַחַמָּה, אֵינָהּ אֲסוּרָה אֶלָּא עִם הָנֵץ הַחַמָּה. רַבִּי יְהוּדָה אוֹמֵר, כָּל הַיּוֹם שֶׁלָּהּ: