you also need to keep in mind that even the best bottles in the world leak, so some of it may not even be being drunken.
generally bottles are very unnatural and are difficult to get water out of. it usually puts rabbits off of drinking enough, or they sit there at an awkward position for ages trying to get enough as they are thirsty. bowls are much more natural for them, so if you could offer one of those, I would. while yes it gets dirty far more quickly which is extra work, as they seem to throw hay in it or poops, its much easier for them to drink from. my indoors make the water dirty, but I'd rather that.
a heavy dog bowl intended for a large dog works, as they can not tip it over. if they do, there are ones that screw onto the side of the cage so they can't move them
they are also sharing a bottle, so they will drink more together, something else to keep in mind.
rabbits drink anywhere from 50-100ml of water per KG of body weight a day, so for rabbits anywhere from 100-600ml of water a day is normal. if you have two 3kg rabbits, it would be expected for them to get through it.
diet also plays apart. a dry diet will require more water, a wetter diet (grass, fresh plants) will likely mean they drink less water due to getting water from the things they eat.
then again, I have two 2-3kg rex's, with a 1 litre bowl, usually that is gone in 2-3 days, if they haven't soiled it already. but they are indoor so they mostly eat hay & forage