TT #11 · Parshas Tazria · Aliyah 1 · Passuk 2
English: Speak to the Israelite people thus: When a woman at childbirth bears a male, she shall be impure seven days; she shall be impure as at the time of her condition of menstrual separation.
Chabad.org — Chumash, English translation + Rashi for Vayikra 12:2
Torah Temimah cites Pesachim 90b.
E-Daf — Pesachim 90b · Shas.org
English: “As in the days of the menstruation of her sickness” compares a woman after childbirth to a menstruant: just as a menstruant immerses at night, so a woman after childbirth immerses at night.
Analogy
The verse’s comparison to niddah supplies the timing of the woman’s immersion at the close of the required period.