Is there any evidence in the Quran or Sunnah confining the condolence period to three days or is it just a habit?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

 We do not know any evidence from the Quran or Sunnah that determine the period of offering condolences. However, the majority of scholars confined it to three days. However, Sufyaan Ath-Thawri and Imaam Abu Haneefah chose the opinion to not offer condolence after the burial.

The scholars concluded this ruling from the Hadeeth "It is not permissible for a woman who believes in Allaah and the Last Day to mourn for any deceased person for more than three days except for her husband; she must mourn for him for four months and ten (days).” [Al-Bukhari and Muslim]

They disliked offering condolence for more than three days unless he was not present. Hence, the wisdom behind offering condolence is to cause the heart of the afflicted one to be tranquil. It most likely becomes tranquil after three days of the affliction. Therefore, it is not recommended to renew his grief.

However, if the one who offers the condolence or the afflicted one did not meet, it is permissible to offer the condolence whenever they meet as there are other  benefits like supplicating for him that is permissible in all times. Among the scholars who adopted this opinion: An-Nawawi in his book Al-Muhaththab, An-Nafraawi in his book Al-Fawaakih Ad-Dawaani and Ibn Muflih in his book Al-Furoo’.

Some jurists did not set any limit to the period of offering condolence. Ibn Muflih in his book Al-Furoo' ascribed this opinion to some Hanbali scholars, and An-Nawawi narrated that it is an opinion of Al-Juwayni according to the Shaafi‘i doctrine.

Allaah Knows best.