I aim to drink around 1.5 litres of water plus a similar amount of other fluids a day including herb teas, a mug or two of coffee, a mug of cocoa, and often soup or a stock cube with a dash of cream. But it varies depending on how thirsty I feel.
If loo gazing isn't an option, you can do a quick hydration test by pinching the skin in your wrist, you're OK if it's elastic and springs back. (I actually learned about this skin pinch when I first had a very sick dog on diuretics; a similar elasticity test by pressing on their gums was another option, and these checks actually saved the life of one of them a couple of years ago.)
Robbity