According to this, 5500-6000 feet/1700 meters
Heritage Concorde
IIRC from one of the previous posts here, the strong differential required also defined the
normal descent/deceleration timing and distance.
Power could only be reduced to 94% (N1 or N2, I forget which) or there would not be enough "bleed" air available to maintain the cabin altitude at TOD.
(although I could have misinterpreted that - it may have had more to do with maintaining the oblique inlet shocks, or hydraulics, or some such.)