I have no idea. I use a 3gal and top off daily to keep it fresher and an eye on consumptionHow often would you have to change out the water in a 5 gallon bucket outside, assuming that the HNs would reduce the amount of wasted water and would probably last a long time.
Again, no idea.... Sorry.... I'm not much help, eh?Also, would you put anything in the outside water to deter algae growth (Apple cider vinegar?, etc...).
I keep mine inside the coop to avoid this problem, plus easier for me to tend to it there.
Also don't think constant acidity from vinegar is good for them.