Lets see, a very rough estimate. A 744 uses about 55 litres a minute taxiing, 210 litres a min with cruise power set and 420 litres a min at climb power.
Estimate the reversers run somewhere between cruise and climb power for 20 seconds, then I expect the burn to be in the region of 100 litres for the reverse power segment? Idle reverse say 50 litres and full reverse 150 litres as a rough guess?