Normally it is suggested to drink from 2 to 4 litres per day. But this include the water that you consume from the food. And the number vary accordingly to your sex, age, amount of physical activity. In general I would say: drink obviously when you are thirsty, and more: being thirsty means that you already lack water in your body. Btw, you are not going to lose weight just by drinking more. Follow a serious diet (which means a little less calories that you need, but not under your basal intake, and with the right balance between nutrients) and do physical activity. That has worked for me.
