The maximum time of nifas (postpartum period) is forty days. If it stops before forty days, starting salah is necessary after ghusl and getting cleanliness. Once the blood stops it is not lawful to pass forty days in the state of uncleanliness. In the question mentioned above, if you start offering salah after completing forty days, while the blood stopped after fifteen days, in this case you will have to offer all those salah which were missed after fifteen days up to forty days.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband