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