Try to put down at least 300 calories per hour (or 15 miles - which ever comes first).
I usually carry two drinks on my rides, Nuun and Gatoraide. The Nuun provides me better hydration and is easier to take than a sweet drink. But I can tell when I am running too low - if I take a long drag on the gatoraide and I get a big boost then I am missing sugars and carbs, at that point I will grab a Guu. Monday I did a 75 mile ride - I had a big slab of Lasagna for lunch about an hour before leaving, that held me well for 2 hours, then I went through 3 trail mix bars, 3 Guu, 1 apple and two bottles of gatoraide and two bottles of Nuun as well as a full hydration pack of ice water (about 900 calories on the ride itself and probably 600 before) during the remainder of the ride, stopping three times on the ride for short stretch and brief walk. Got back and was tired but not bonked. Wife ordered a pile of Chinese food so I pigged out on recovery food when I got back. Averaged well over 17MPH for the ride.