What quantity of dessicant agent It is known that dessicant agents such as magnesium sulfate can be used in order to trap water. But I was not able to find what quantity has to be added ideally compared to the amount of water that would be trapped. I know you can put it in excess but it could be a waste sometimes.
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.