There isn't a "general amount" because GHB can be dissolved at all sorts of concentrations. Sometimes it is 1 gm per mL, sometimes more, sometimes it's less.
The only way to know for certain is measure out a known volume of liquid, boil off the water, and weigh the residue of solid rocky sodium GHB that should remain. Then you can do the math and have a pretty surefire figure on how many grams of sodium GHB is dissolved in 1mL.
The other thing that is an option (but may not be reliable) is to ask the person who's giving it to you. Ask them what a good dose is, then take half that.
Stick on the side of taking less rather than more. GHB has a very steep dose curve. For an average individual, 1gm might have no or only mild effects, 2g good, 3g too much, and at 4g you're unconscious on the floor.
If you're a newbie to GHB take about 1.5 - 2gm (less than half a capful) your first time, assuming you weigh about 60kg. You can always take a little more if you like, GHB kicks in pretty quickly, but keep in mind if you take too much you will feel very unpleasant.
