It really depends on your system. If you were using a hanging hop bag and a bottom drain on the kettle you'd have close to 0 gal lost to kettle break etc. For my system it does depend on the amount of hops, but there is also a small amount that gets left behind no matter what (I've worked to eliminate as much of that as possible.) It is just trial and error per system.
"People should not be afraid of their governments. Governments should be afraid of their people." - V
Secondary: Shady Lord RIS, Water to Barleywine, Pumpkin wine, burnt mead
Kegged: Crappy infected mild
Bottles: Apfelwein, 999 Barleywine, Oatmeal Stout, Robust Porter, Robust smoked porter, Simcoe Smash