What quantity of dessicant agent
You add dissicant until it suspends easily in the solvent when you swirl the flask. Some people say: "Until it snows".

When you add a bit of magnesium sulfate or other fine powdered dissicant, it forms a lump and goes to the bottom. 
Many times it just sticks to the bottom when you swirl the flask. 
You add more solid and swirl until it suspends and then slowly falls to the bottom, like it is snowing. At that point it is taking no more water and it has no sense to add more.