It needs to be mentioned that, in Islamic Law, blood from the womb only takes the ruling of postnatal blood if a child is delivered, alive or stillborn, that shows some human features. If a pregnancy ends in miscarriage before that time and only a lump of flesh appears, then the associated blood will not take the ruling of postnatal bleeding. Usually, this is the case when a miscarriage occurs during the first eighty or ninety days of pregnancy.
Now I will turn to your question of the minimum duration of postnatal bleeding.
Scholars differ on this issue. Many say that the minimum duration of postnatal bleeding is a single instance of blood. This is the position of the Mālikī, Shāfī`ī, and Hanbalī schools of law. It is also one of the opinions expressed by Abū Hanīfah.
There are two other opinions that have been narrated from Abū Hanīfah. One is that the minimum duration of postnatal bleeding is eleven days. The other is that the minimum duration is twenty-five days.
There is a narration from Ahmad b. Hanbal that the minimum duration of postnatal bleeding is one full day. Other scholars have expressed views like three days or four days.
The truth is that there is no minimum time period for postnatal bleeding. A woman might only experience a single instance of blood after childbirth. If that is the case, she should immediately bathe and then resume her prayers.
It may be that she sees no postnatal blood whatsoever. Though this is rare, it has been known to happen. In this case, scholars differ as to whether she has to bathe at all before offering prayers.
Some argue that she should bathe before she can pray, and this is due to the childbirth itself. It seems that he stronger opinion is that bathing is not required for her to be allowed to pray, since the bath becomes incumbent due to the flow of blood from her private area.
And Allah knows best.