What is the duration of menstruation, post-natal bleeding (nifas), and prayer based on the mentioned symptoms?

The first drop of blood is considered menstruation when it is continuous with subsequent blood. If the bleeding stops after four days and you see purity by the white discharge (al-Qassah al-Bayda') or dryness, then you must perform ghusl (ritual bath) and pray. Any brownish or yellowish discharge you see after that is not considered menstruation. So, the duration of your menstruation is what precedes the complete cessation of bleeding and the sight of one of the two signs of purity.