This is about all I can find.
Ground Operations
Fuel can be saved by minimizing APU utilization.
Average APU fuel flow rate under normal operation is
105kg per hour on the ground.
APU Operation During Flight
For APU operation during flight, increase fuel flow
acording to the following table. These increments
include the APU fuel flow and the effect of increased
drag from the APU door.
Pressure
Altitude........ APU FUEL FLOW (KG/HR)
(1000ft)
39................ 70
35................ 75
31................ 85
25................ 95
20................ 105
15................ 110
10................ 125
5.................. 135
page 2.1.1
Source
Flight Planning and Performance Manual B757-200 RB 211-535
CAA July 30 2010
B757 Systems Manual states....
APU Electrical power is available throughout thre airplane operating envelope. In flight, APU bleed air is available up to approximately 17,000ft.
Hope this helps. A search on the web may find more.