According to most scholars among the Hanafis, Shafi’is, and Hanbalis, a waqf must be perpetual. However, according to the Malikis (and one narration of Imam Abu Yusuf) waqf can be made for a temporary period. This means that once the stipulated period has ended, the waqf asset will be return to the original owner if he/she is still alive. If not, then it will be returned to their legal heirs.
The Malikis argue that only some waqf can only be perpetual such as mosques.
AAOIFI issued a standard allowing the structure of temporary waqf.
And Allah knows best!
(Mufti) Billal Omarjee