Let your body tell you. If you're thirsty, obviously drink

Same with the high-fiber diet. If this is a new WOE for you, you will need extra water until your body's used to the change. You will know if you're not getting enough water with a high-fiber diet because you will suddenly be constipated (I found this out personally...funny how our bodies "talk" to us.)
Local temperature, humidity levels and personal activity levels play such large parts in this. I've never agreed with, "Drink X amount of water per day; any more or any less is bad" theories. I'm in the sweltering hot tropics. In the height of summer, most everyone I know has consumed 100 ounces of water by lunch/early afternoon. When the weather changes and it cools off, we drink less.