How hard you're riding has a lot to do with calorie needs, too. In the summer, when I race, I'll try to eat a gel every 30-40 minutes and drink a weak mix of gatorade & extra salt in between gels. Usually I'll put away a bottle of fluid every 45 min-1 hr. When I'm riding easy in the winter time, I may drink a bottle of weak gatorade over the space of 2 hours and it sustains me just fine.