Enough so that you need to pee about every 3 hours. That's the gold standard. If you're not riding that long, don't worry about it. You can be about 1-1/2 quarts low before it begins to affect your performance. Color of pee is immaterial - depends on what you've eaten to a large extent. I ride with a guy who drinks about 12 oz. of water on a 60 mile ride. Another guy sweats so much one thinks it's raining when riding his wheel.
Isotonic makes no difference. Water's H2O and that's that. Electrolytes are a separate story. The need for them varies as much or more than the need for water. I drink plain water and take enough Endurolyte capsules that I stay a little thirsty. But that's a long ride tactic, 3 hours and up.
Watch your forearms. If they dry up, you're about to have a medical problem. Pinch the skin on the back of your hand. If it stands up, you're dehydrated.
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Results matter