The rule of thumb is about 64 fluid ounces (1.8 liters) a day. More if it's hot outside or if you're really active and you sweat a lot. Less when it's cold or if you're not active.
And actual thirst is a good indicator too.
A better indicator is the color of your urine. Sorry yes, this is a little disgusting but it's what we medical people tell patients. If your urine is a light clear yellow color, then you're getting enough water. The darker it is, the more dehydrated you are, and you need to drink more.
And it's preferred to drink your water with meals, as this improves the absorption and also helps with digestion. When you drink water on an empty stomach it tends to just pass through your stomach, intestines, and back out again.
So drink water with meals and go by the color of your pee.
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That which does not kill us makes us stranger.