Narrated Abdullah ibn Amr ibn al-‘As:
Wahb ibn Munabbih said: Abdullah ibn Amr asked the Prophet (ﷺ); In how many days should one complete the recitation of the Qur’an? He said: In forty days. He then said: In one month. He again said: In twenty days. He then said: In fifteen days. He then said: In ten days. Finally he said: In seven days.
[Graded Saheeh by Al Albani in Saheeh Sunan Abi Dawood no. 195]
Ibn Qudamah rahimahullah said: “it is disliked to delay reading the Quran for more than forty days because the Prophet sallalahu alayhi wa salam was asked by Abdullah ibn Amr (radi allahu anhu) in how many days should the Quran be completed? He said forty days and then he said a month and then he said 20 days and then he said fifteen days and then he said ten days and then he said seven.
[reported by Abu Dawood]
Ahmad [ibn Hanbal] said: “the most I have heard about completing the Quran is forty days”
This is because delaying it more than that causes forgetfulness of the Quran and taking it lackadaisical. So what we mentioned is more correct and this is if he does not have an excuse. As for if he has an excuse, there is room/space for him’.
And Ishaaq ibn ar-Rahawayha rahimahullah said:
“As for the reciter of the Quran, whether through memory or looking at the Mushaf, it is recommended for him not to exceed forty days until he finishes the Quran once…[Then he mentions the Hadith above]
[Masaail Ishaaq ibn Rahawayha no. 3525]
So it is not a must but only disliked to go beyond that as the shaikh mentioned. Imam Suyuti has a discussion of the issue in his book Itqaan fi uloom al-Quran and he mentions evidence of the salaf and scholars proving that it is allowed to go beyond forty days, but it is disliked.
And Allah knows best
Faisal ibn Abdul Qaadir ibn Hassan