If a woman’s nifaas ends before forty days, she should do ghusl and pray and fast

The majority of scholars, including the four imams, are of
the view that there is no minimum limit for nifaas. Whenever a woman becomes
pure from nifaas, she has to do ghusl and pray and fast, even if that is
before forty days have passed since the birth. Because no precise definition
has been narrated in sharee’ah with regard to the minimum length of nifaas,
so the matter should be based on experience, which shows that nifaas may be
very short or it may be long. – Ibn Qudaamah in al-Mughni, 1/428.
Some of the scholars narrated that there was consensus on this point.
Al-Tirmidhi (may Allaah have mercy on him) said: The scholars among the
companions of the Prophet (peace and blessings of Allaah be upon him),
the Taabi’een and those who came after them are agreed that a woman in
nifaas should stop praying for forty days, unless she sees that she has
become pure before that, in which case she should do ghusl and pray. 

See al-Majmoo’ by al-Nawawi, 2/541. 

Shaykh Ibn Baaz (15/195)
was asked: is it permissible for a woman in nifaas to pray, fast and do Hajj
before forty days if she becomes pure (i.e., if the bleeding stops)? 

He replied: yes it is permissible for her to fast, pray and
do Hajj and ‘umrah, and it is permissible for her husband to have
intercourse with her within forty days, if she becomes pure. If she becomes
pure after twenty days, she should do ghusl and pray and fast, and she is
permissible for her husband. The report from ‘Uthmaan ibn Abi’l-Aas which
says that he regarded that as makrooh is to be understood as meaning
disliked but not forbidden; this is his ijtihaad, but there is no evidence
for that. 

The correct view is that there is nothing wrong with that if
the woman becomes pure before forty days. If the bleeding resumes within
forty days, then the correct view is that she should regard it as nifaas
within the forty-day period, but her fasts, prayers and Hajj at the time
when she was pure are still valid and she does not have to repeat any of
them, so long as they were done when she was pure. 

It says in Fataawa al-Lajnah al-Daa’imah (5/458): 

If a woman in nifaas
sees that she has become pure before forty days are over, then she should do
ghusl and pray and fast, and her husband may have intercourse with her. 

The Standing Committee (10/155) was asked about a woman who
gave birth seven days before Ramadaan, then she became pure and fasted
Ramadaan. They replied: If the matter is as described and she fasted
Ramadaan at a time when she was pure, then her fast is valid and she does
not have to make it up.