[The ruling on intermediate purity during nifās]

Answer: According to the school, as in Sharḥ al-Azhār, if the cessation of blood continues for ten full days—from time to time— it is purity and not nifās. Based on that, the woman’s fasting during the twenty days—which were a time of cessation—is valid fasting, so it is not obligatory upon her to make it up [sic]. Moreover, some scholars say that it is nifās if it occurs within forty days, but what is established in the school is as we have stated.
What indicates the soundness of what the scholars of the school mentioned is that ten days constitute valid purity, which is their minimum purity. Therefore, if the blood of nifās ceases for ten days or more, the necessary ruling is that it is purity.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1