Spiritual Retreat: The Duration of Iʿtikaf and Fasting
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1725
The minimum duration of iʿtikāf is three days; less than three days is incorrect. However, there is no maximum limit, as mentioned in Ruling 1721.
v. A muʿtakif must fast during the days of iʿtikāf.
-Ayatullah Sistani, Practical Laws of Islam, Spiritual Retreat (Iʿtikāf)