I don’t even think this is the right metric to use.
You aren’t putting a lithium battery fire out with water. You’re just keeping it at bay until the energy is all used up. The more energy, the longer it’ll take.
We might need new ways to deal with these fires, but it’s not like we can completely submerge a semi in water.
I wonder how encasing the object in a fire retardant foam would behave, although we gotta think about the toxicity of that too.
Edit: I wonder if you could even calculate the amount of water you’d need to hold it off upfront based off the battery size and current charge.