I don't know the actual amount but it's not much. The bigger problem is when they play with the water, spill some, or drip. Also a slightly deformed bottle can drip a little. You can safely go two full days (unless bottle is broken, but you'd know), even 3. I've had a 3 day trip that inadvertently turned into 4 due to a delay. Both birds still had water in their bottles left but Truman had less than a quarter bottle. Furthermore he didn't seem to want it. When I poured him fresh water he drank eagerly.
Sometimes when the birds take a shower out of their bottle, that uses up about 1/3 of the water at a time.
Let the bird that doesn't know how to use the bottle watch the bird that does. They model off each other much quicker.









