Reference: ad-Durar an-Naadhirah fil-Fataawa al-Mu’aasirah – Page 380;
ad-Da’wah 1550 – Rabee’ al-Awwal 1417AH
Question: Is [engaging in] i’tikaaf in other than Ramadhaan permissible? And what is its [legislated] duration?
Response: [Engaging in] i’tikaaf is permissible at any time during Ramadhaan and other than it; However, [engaging in] i’tikaaf during Ramadhaan is better [as it is] in accordance with [what] the Prophet (sal-Allaahu ‘alayhi wa sallam) [did], as well as for the superiority of the time (the month of Ramadhaan). And [engaging in i’tikaaf] does not have any specified duration, [however] the longer the duration [one engages in i’tikaaf] the better, so long as [doing so] does not occupy him from that which is more important than it from [other] acts of worship as well as seeking provision (i.e. maintaining one’s job).