There is no minimum duration for nifaas. If a woman’s nifaas (post-partum bleeding) ends after giving birth, even after only a few days, then she should do ghusl and pray and fast.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:
When a woman becomes pure [i.e., the bleeding ends], even if it is only one day or a few days after giving birth, then she is taahir (pure) and she must pray; her fasting is valid and it is permissible for her husband to have intercourse with her.
End quote from Fataawa Noor ‘ala ad-Darb by Ibn ‘Uthaymeen.
It may be determined whether a woman has become pure – following menses or nifaas (postpartum bleeding) – by one of two signs:
For more information, please see the answer to question no. 156224.
These few drops of blood that appeared after becoming totally pure from nifaas are not regarded as being part of nifaas. Based on that, the woman should pray and fast in that case.
In Fataawa al-Lajnah ad-Daa’imah (vol. 2, 4/259), it says:
His wife gave birth on the ninth day of Ramadan, and nine days after giving birth the bleeding stopped, so she did ghusl and started to pray and fast, but she noticed when night came that there were a few drops of blood, but she did not see anything like that during the day. What is the ruling on that, and are her prayer and fasting valid?
Answer: If this woman saw a complete cessation of bleeding or the white discharge, then her prayer and fasting are valid, because she comes under the same ruling as women who are taahir (pure, i.e., not menstruating or bleeding postpartum). What she sees of a few drops of blood at night is not regarded as nifaas and is not described as blood, so it does not come under the ruling on nifaas. End quote.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked:
There is a woman who, after two months of nifaas and after becoming pure, began to find some small drops of blood. Should she stop fasting and stop praying? Or what should she do?
Answer: If this woman became pure and definitely saw the tuhr indicating that menses or nifaas had ended – by which I mean the emission of the white discharge, which is well known to women – then anything that appears after the tuhr of brownish or yellowish discharge, or spots of blood, or wetness, none of this is menses, so it does not prevent her from praying and it does not prevent her from fasting, and it does not prevent a man from having intercourse with his wife, because it is not menses. Umm ‘Atiyyah said: We used to regard the yellowish and brownish discharge as not being of any significance. Narrated by al-Bukhaari. Abu Daawood added: …after the tuhr. Its isnad is saheeh.
Based on that, we say: Anything of that nature that happens after the tuhr has definitely occurred does not matter, and does not prevent the woman from praying or fasting or being intimate with her husband. But she should not rush until she sees the tuhr, because some women, when the bleeding stops, rush to do ghusl before seeing the tuhr. Hence the women of the Sahaabah would send their pieces of cloth – meaning the cotton on which there was blood – to Umm al-Mu’mineen ‘Aa’ishah (may Allah be pleased with her), and she would say to them: Do not be hasty, until you see the white discharge. End quote.
And Allah knows best.