In my experience, weaning an infant from the isolette is not a function or weight or gestational age. Rather, it is accomplished by observing the infant's requirement for external thermoregulatory support. As the infant's requirement for external warmth decreases, the environmental temperature of the isolette will decrease. When it drops below 30 degees C, the infant may be ready for an open crib trial. You must also factor in the infant's growth and ability to tolerate the increased visual and auditory stimulation that comes from taking them out of the isolette. If they aren't growing adequately, or are excessively agitated, you may want to leave them in the protected environment of the isolette to decrease their energy expenditure and promote improved growth.
That said, there is an increasing body of evidence that infant nurturing leads to improved growth and development. It is important to encourage human contact and mother/infant interaction as soon as the infant is stable enough to tolerate the stimulation. This may mean taking them from the isolette for feeding, kangaroo care, etc. even before they are physiologically ready to maintain their own body temp.
There is my two cents...hope it helps.