You will save fuel weight of at least 25% of the weight of the water not being carried on longhaul (on B744, I would think B777 and A380 would be similar). Use weigh, not liters/volume.
From flight plan: SYD-PVG (10:08 flight time) saving of 29.6% of the weight not carried.
Longer flight of 14:14 hours shows 39.6%