Nachschlagewerk zu Shabbat 9:11

If he emitted thick drops from his member, he is unclean, the words of Rabbi Elazar Hisma. If one had sexual dreams in the night and arose and found his flesh heated, he is unclean. If a woman discharged semen on the third day, she is clean, the words of Rabbi Elazar ben Azariah. Rabbi Ishmael says: sometimes there are four time periods, and sometimes five, and sometimes six. Rabbi Akiva says: there are always five.