The minimum length of time for iʿtikāf is two nights and three days; less than that is not correct. There is no maximum limit. There is no problem in including the first or the fourth night in the intention of iʿtikāf. If a person is a muʿtakif [the term given to someone who is performing iʿtikāf] for five full days, he must also be a muʿtakif on the sixth day.