How much water shoudl you drink while on lithium treatment?

Consistent. Whatever your normal fluid intake is during the beginning of treatment, when your doctor is checking lithium levels, is what you should continue once your dose has stabilized. Your doctor should explain to you all the things that can affect your lithium levels (dehydration, over-hydration, certain drugs, etc.). The important thing is to avoid extremes. Talk to your doc for specifics.