There is no Defined Minimum Period for Postnatal Bleeding
Question :
Is it permissible for the woman who has postnatal bleeding to pray and fast if she becomes clean before forty days?
Answer:
If a woman with postnatal bleeding becomes clean before forty days, it is an obligation upon her to perform Ghusl, pray and fast in Ramadhan, and it becomes permissible for her husband to have sexual intercourse with her, according to the consensus of the scholars. There is no defined minimum period for postnatal bleeding. And Allah is the Granter of success.
Source:
Ibn Baz
Fatawa Islamiyah, Vol. 2 Page 120