There is no minimum for nifaas. A woman may become pure from nifaas one or two days after giving birth. When a woman becomes pure from nifaas, she has to do ghusl and start praying, even if that is before forty days have passed.
Al-Tirmidhi (may Allaah have mercy on him) said: The scholars among the companions of the Prophet (peace and blessings of Allaah be upon him), Taabi’een and those who came after them are agreed that the woman who is bleeding after childbirth must refrain from praying for forty days, unless she sees the tuhr (clear discharge indicating that the nifaas is over) before that, in which case she should do ghusl and pray. End quote.
It says in Fataawa al-Lajnah al-Daa’imah (5/484): If a woman who is bleeding following childbirth sees the tuhr before forty days have passed, she should do ghusl and pray and fast, and her husband may have intercourse with her. End quote.
As you did not pray for seven days, even though you knew that when a woman becomes pure she has to pray, then you have to make up the days that you missed. The way in which you should make them up is by praying five prayers (Fajr, Zuhr, ‘Asr, Maghrib and ‘Isha’) for the first day, then five prayers for the second day, and so on, until you have made up the seven days. This should be done immediately and it is not permissible to delay it, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever forgets a prayer, let him offer it as soon as he remembers, for there is no expiation for it other than that. ‘And perform As‑Salaah (Iqaamat‑as‑Salaah) for My remembrance’ [Ta-Ha 20:14].”
Narrated by al-Bukhaari (597) and Muslim (684).
If it is difficult for you to make up these prayers all together at one time, then you can offer some of them, then rest, then pray some more, until you have made them all up, even if that takes several days, so as to avoid hardship and difficulty.
And Allaah knows best.