I usually drink 2 large cups of weak tea in the morning, one at lunch, 2 at tea time, one late evening. In between, I have a glass of water by me at the computer, and refill it if it disappears. I have a glass of water by me at night in case I wake up hot.
I have done this ever since I had a kidney stone (7 years ago) and if I do not do it, I get a headache and constipated. The most effective way of preventing further kidney stones seems to be to drink plenty, and so far, so good.
The amount of water I drink varies - I need very little extra in the winter, and loads in the summer.
I never really feel hugely thirsty, which is probably why I can ignore it and end up with a headache. It is amazing how a glass of water will disappear, though!