If I have a pump apart, and just want to clean the impeller, I use straight vinegar.
Otherwise, use as much as you want......the more you use, the faster it cleans. I usually take a 5 gal bucket, put the powerhead in the bucket. Fill with warm water to just about the point where the pump would suck air. The start the pump and top off with vinegar. Usually takes about a day, if the pump has a ton of coralline on the outside, after 3-4 hours, it has softened up, and I scrap off the excess.