I did the CTS climbing power video today and noticed the trainer said that the body can exercise 1.5 - 2 hours without nourishment. Anything beyond that they suggest gels or energy drinks. Since it was a power video I imagine she meant to maintain peak power.
Carmichael is big into loading with carbs and hydration before, during and after and I've found that to be a good idea. His book has the formulas based on weight and ride length and intensity. Since his latest stuff is based on time crunched cyclists the ratios of carbs, proteins and fats is skewed heavily to carbs.
I follow his advice and it seems to work. As for eating, sometimes I can go hours without consuming big calories. Other times I have to have a clif bar or banana. Long rides I'm probably having Infinit drinks at 280-300 calories each. One per 1.5 hours or so.
Listen to your body. Something I learned while I was losing 50 lbs. If you're hungry, eat. The body seems to know what's right. GL