משנה אויף ישעיהו 64:5

Shammai says: for all women it suffices [to reckon] their [period of uncleanness from their time [of discovering the flow]. Hillel ruled: [their period of uncleanness is to be reckoned retroactively] from the [previous] examination to the [last] examination, even if this was many days. The sages say: [the law is] not like the words of these or the words of those, but [the women are deemed to have been unclean] during [the preceding] twenty-four hours when this lessens the period from the [previous] examination to the [last] examination, and during the period from the [previous] examination to the [last] examination when this lessens the period of twenty-four hours. For any woman who has a regular period it suffices [to reckon her period of uncleanness from] the time she discovers the flow. And if a woman uses rags when she has marital intercourse, this is like an examination which lessens either the period of the [past] twenty-four hours or the period from the [previous] examination to the [last] examination.

Although though they said [that for a woman who has a regular period] it suffices to reckon her period of uncleanness from the time she observes a flow, she should nevertheless examine herself [regularly], except for a menstruant or one who is sitting over pure blood. She should also use testing-rags when she has marital intercourse except when she is sitting over pure blood or when she is a virgin whose blood is clean. And twice [daily] she should examine herself: in the morning and at the [evening] twilight, and also when she is about to have sexual relations. Priestly women are subject to an additional restriction [for they should examine themselves] when they are going to eat terumah. Rabbi Judah said: [these must examine themselves] also after they have concluded eating terumah.