Pubic hair longer than a grain of rice
In the name of Allah, the most Beneficent, the most Merciful.
Saaiduna Abu Huraira Radiallahu Anhu narrates that the Prophet of Allah Sallallahu Alahi Wasalam said “Five practices are of Fitra: circumcision, shaving the pubic hair, clipping the nails and cutting the moustache short.” (Sahih Bukhari p.874 v.2)
It is recommended to remove the pubic hair every seven days. It is also permissible to delay this to fifteen days, beyond forty days is Makruh Tahrimi and sinful. (Bashti Zewar p.479)
The hair should not be left longer than forty days. This is because of the hadith narrated by Saaiduna Anas bin Malik Radiallahu Anhu who said, “He set us a time limit of no more than forty days for trimming the moustache, clipping the nails, plucking of armpit hairs and shaving the pubic hair.” (Sahih Bukhari & Sahih Muslim)
With regards to your question I have not come across a view that the pubic hair has to be the length of a grain of rice for it to become necessary for removal.
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham