I started off with teaspoon increments when I first tried kava, but now I start with one level tablespoon to a little less than a cup of ice cold water and repeat every 20-30 minutes. I usually feel full effects at 3-4 tablespoons.
Edit: I don't measure my water, but it usually ends up being 4-5 quick gulps because I don't like to draw the process out. Also, it helps to have a strong chaser such as crystallized ginger or a spritz of Listerine.