There is no minimum for Nifas (postpartum bleeding). A woman may become pure from Nifas one or two days after giving birth. When a woman becomes pure from Nifas, she has to do Ghusl and start praying, even if that is before forty days have passed. 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 correct scholarly view is that if a woman miscarries a foetus that was fully formed, then she should stop praying and fasting because this is postpartum bleeding. If the foetus was not fully formed, then her blood is irregular bleeding and she should not stop praying and fasting.