See, now I am going on Frankie and Belle who have a 1 litre water bottle each. and there would be a little water at the end of day two, probabely enough to last over night to day 3, but I don't like it getting that low.
I suppose If I think of Granite, he is a single bun and his water bottle doesn't go down as quickly, I would estimate definately less than 200ml
Where as Arthur would go through a dog bowl(approx 1 ltr, if a bit more) in a 24-36 hour peroid, but he is a large bunny.
Personally, if they always have the option of clean fresh water I wouldn't worry too much, just know what is normal for your bunny.