Parashas Metzora · Passuk 15:18
Chabad.org — English translation + Rashi
Torah Temima points to Niddah 42a.
English: “And they shall be impure until evening” — this applies when she does not turn about; but if she turns about, for three full days she is forbidden to eat terumah, because it is impossible that she not emit.
Extended impurity when emission continues
Chazal extend the woman’s restriction when continued emission of seed is expected over three days.