What is the ruling if nifaas blood after childbirth discontinues way before the typical 40 day postpartum period ends?

There is no minimum or maximum limit to the length of time nifaas clears the body. So, the woman can resume normal activities once the bleeding stops. If after 40 days, the blood no longer looks like the nifaas blood and has become lighter in color, it can then be treated as istihatha and the woman can resume normalcy.

Narrated Umm Salamah (ra):
“During the lifetime of the Prophet (pbuh), the women having bleeding after delivery (postnatal puerperal blood) would refrain (from prayer) for forty days.“