Purity: Nifas Exceeding Ten Days
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 507:
If a woman’s nifās exceeds ten days, then in the event that she has a habit of duration for ḥayḍ, the same number of days of her habit is nifās and the rest is istiḥāḍah. If she does not have a habit, it is nifās for the duration of ten days and the rest is istiḥāḍah. If she has forgotten her habit, she must consider her habit to be the highest number of days that she deems probable. Furthermore, the recommended precaution is that a woman with a habit should perform the things required for a mustaḥāḍah and refrain from doing the things that are unlawful for a nufasāʾ, starting from the day after her habit. A woman who does not have a habit should do this after the tenth day until the eighteenth day after childbirth.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)