Probably about 4-5" -that's for the tip of the tube above the cage floor or from the top of any substrate you put in there- to accommodate all ages/sizes of rats. If the tip contacts any substrate, it will continuously wick out the water. Water bottles usually are sold with a basic attachment thingy- so wait until you have the bottle & holder in hand to measure.
It would probably be better to just put a hole for the tube to poke inside, & attach the bottle outside, since rats (& other rodents) will use it to climb on if it's available, & when it bangs against the side for a while, it will drip more. Rats will be rats...![]()