I recommend a 20-40% change. Best thing to do is fill a 5 gallon bucket with tap water, dechlorinate, and put an airstone in the bucket on the first day then after 72 hours it'll be nice and aged for your water change. You don't need to do that, but its nice.
The fenbendazole degrades quickly in the presence of light and oxygen, so the amount of water changed isn't too critical. Some people don't do one at all
It works beautifully, but if you have nerite snails you'll want to take them out of the tank first, as this can kill them as well. The hydra pull in their tentacles, day 2 there might be a few still clinging to the plants, day 3 they're gone.