with my hydrometer, I need about 3 or 4 ounces (I'm guessing here, just eyeballing it).
Actually, I just got a plastic hydrometer test jar for like 3 bucks at the LHBS, but I used to just use the little plastic tube the hydrometer comes in, and it takes a little bit more beer to get a reading with the jar (5 ounces or so?). I used to just use the little tube the hydrometer came in, and that worked alright, but the hydrometer kinda sticks to the side of the tube. if you keep fiddling around with it you can get it to float pretty much freely. It works in a pinch anyway.