You don't need a lot. Just add enough water to cover the powder, and so that the solution/mixture is not too thick or viscuous. You can eyeball it pretty well. More water will extract hydrocodone, but it was also solvate more APAP as well. So it is best to use as little water as possible.Apparently hydrocodone solubulity in water is 60mg /ml (https://groups.google.com/d/msg/alt.drugs/JTt_igA7u0w/lvjhsJZ9V3gJ). So I would use just enough water to make the physical process of filtering out the APAP simple. If you use too little, you'll probably wind up with a lower yield. If you use too much, you will get more APAP than you want. Just use good judgment, and if you're really worried, do multiple filters. The clearer the solution eventually is, the less APAP there is.