The scholars differed as to the minimum length of time for i’tikaaf.
The majority of scholars are of the view that the minimum length of time is a moment. This is the view of Abu Haneefah and Ahmad. See al-Durr al-Mukhtaar (1/445); al-Majmoo’, 6/489; al-Insaaf, 7/566.
Al-Nawawi said in al-Majmoo’, 6/514:
With regard to the minimum length of time for i’tikaaf, the majority stipulated that it must be observed in the mosque, and that it is permissible to do a lot or a little, even a hour or a moment.
They quoted several reports as evidence for that:
1 – That i’tikaaf in Arabic means staying, and the word may be applied to a long period of time or a short one; there is no report in sharee’ah that defines it as being a specific length of time.
Ibn Hazm said: I’tikaaf in the language of the Arabs means staying… any stay in the mosque for the sake of Allaah with the intention of drawing closer to Him is i’tikaaf… whether that is for a short time or a long time, because the Qur’aan and Sunnah do not specify any number or length of time. Al-Muhalla, 5/179
2 – Ibn Abi Shaybah narrated that Ya’la ibn Umayyah (may Allaah be pleased with him) said: I will stay in the mosque for an hour, and I am only staying to observe i’tikaaf. Ibn Hazm quoted this as evidence in al-Muhalla (5/179); it was quoted without comment by al-Haafiz in al-Fath. “Hour” here refers to a period of time but it does not mean an hour in the modern sense of sixty minutes.
Some scholars are of the opinion that the minimum length of i’tikaaf is one day. This was narrated from Abu Haneefah and was the view of some of the Maalikis. Shaykh Ibn Baaz said in Majmoo’ al-Fataawa (15/441):
I’tikaaf means staying in the mosque to worship Allaah, whether that is for a long time or a short time, because as far as I know there is no report to indicate a set time, whether one or two days or more. This is an act of worship which is prescribed in Islam unless one vows to do it, in which case it becomes obligatory. This applies equally to men and women.