It's recommended that you drink between 3 and 4 litres of water per day. Closer to 4 if you can, the more you drink the less light headed you will be and you will also lose more weight. I noticed greater weight loss the weeks I drank the 4litres per day than those when I had 3.
Advic is that you shouldn't exceed 4 as you deplete your body of minerals and salt which causes other issues.
Good luck! Xx