The Fayetteville Observer reports that Superior Court Judge James Webb told lawyers Stewart they must make the request by May.
The judge also denied a request by prosecutors to start the murder trial early this fall. Prosecutors plan to seek the death penalty.
The 46-year-old Stewart is accused of killing seven patients and a nurse. Stewart's estranged wife worked at the nursing home but was unhurt after locking herself in a room.
The rampage ended after Stewart and a Carthage police officer were wounded after exchanging gunfire.