Q: How often should I recite the entire Qur'an?
A: It is recommended to complete reciting the Qur'an in its entirety every seven days.
ʿAbdullāh, the son of Imam Ahmad said, that his father would recite the Qur'an from the muṣḥaf in its entirety every seven days and would rarely leave it off.
This is based on what the Prophet (peace be upon him) said to ʿAbdullāh b, Amr, “Read the Qur'an within every seven days and do not do more than that” (Abu Dawud).
If it is read within every three days, this is also good. This is based on ʿAbdullāh b. Amr saying to the Prophet, “O Messenger of Allāh, I have the ability to do more”. He responded by saying, “Read it every three days” (Abu Dawud).
There is also no problem reading more on special occasions and in special locations like Ramadan or while visiting Makkah.
Delaying a complete recitation beyond 40 days without excuse is makrūh/abhorred because it leads to forgetting and neglecting it.
It becomes impermissible if you fear that you will forget it due to not reciting within that time frame.
Source: Kashshaf al-Qina'