How much hydration is enough hydration during long rides?
When I first started doing longer distance rides, the rule-of-thumb I was told was &quot;drink a bottle every hour.&quot;
Obviously, that's a very rough figure, and it's going to depend on a lot of factors, including how much you're sweating (a combination of workload and heat+humidity), how much you weigh, and just generally how much fluid you personally need.  The first time I did a century (160km/100mi) I was loaded touring and it was very hot; that day I drank 9l of water on the road, and still needed to keep rehydrating that evening.
But to get to my point.  After many years of riding, I've found one of the best indicators that I'm getting adequate hydration is that I keep regularly needing to urinate; if I find I'm going more than 2.5h without needing to relieve myself, I'm getting dehydrated.  Again, there's a certain amount of knowing your own body here, and my numbers won't be the same as yours.  But if you're doing a long ride, you're going to need nutrition; metabolizing nutrients (especially protein) produces water and metabolic waste products you need to get rid of.  If you're not doing so regularly, it means that your body is starting to hoard water and you need to drink more.