20-25 miles, I'll bring 2 water bottles. If it's hot, I'll have 0-calories electrolyte powder in them.
Any longer than that, I might use Gatorade Endurance in my bottles (has carbs/calories), and/or bring a granola bar.
I tend to feel the need for calories on rides 30 miles or longer (ie 2 hours or more). I also notice that when I am mindful of this (eat something an hour before the ride, take carbs during the ride), I get home and I'm not looking to raid the fridge.
Obviously athletes have very different nutritional needs, but as I ride for fun and fitness, I'm figuring out what my body needs depending on the length of the ride, and to some extent how dang hot it is outside.