Quite possibly any altitude!
Once fuel burn at low altitude is not an issue a prop/fan driven electric aircraft will have much greater practical and economic flexibility.
The need for a jet and a turboprop for cold air at altitude to be efficient is gone, ie , a much greater range of efficient cruise altitudes!
My guess would be to make it efficient from 15 000 feet to 35 000 feet
And the level from FL 260 to say FL 320 is mainly unused for cruise,