After killing here for a couple hours and still not being graced with his presence, I can say for certain that the respawn time on the place holder "Skeletal Duke" is 27 minutes.
Cheers,
Xristophero
Spawns within the pyramid/crypt at (-1310, 245, -100) with skeletal duke as PHer. He spawns in the NW room of the 4-rooms area.