If a woman's
habit is 8 days, but she stops bleeding after 6 days, is she allowed to resume
relations with her husband after having a bath, or should she wait till the 8th
day and then only resume relations?

Answer: 

Once the bleeding stops after 6 days, it is Waajib upon her to take Ghusl and perform Salaah. However, she is not permitted to engage in relations until her habit of 8 days terminates, as it is possible for the menstrual bleeding to return after the 6th day.

ولو انقطع دمها دون عادتها يكره قربانها وان اغتسلت حتي تمضي عادتها - - - - -الخ )الفتاوي الهنديه ص  24 ج 1 )

Yes, if a woman has a habit of less than 10 days (for example 8 days) and on completion of the 8th day, her menstrual bleeding terminates, then she is permitted to engage in relations with her husband after taking a bath or after a Salaah time elapses.

AND ALLAH TA’ALA ALONE IN HIS INFINITE KNOWLEDGE KNOWS BEST

ANSWERED BY: Mufti Mohammed Desai 

CHECKED AND APPROVED BY: Mufti Muhammed Saeed Motara Saheb D.B 

Date: 08 Safar 1441 

English Date: 07 October 2019