I think everyone is different, and you have to listen to your body. I have a screaming metabolism (I'm one of those lucky ones who eat all day and don't gain a pound). When I first got into cycling, I made the mistake of eating only a banana before a 30 mile ride, and barely made it home. My current routine is two packets of instant oatmeal mixed with a big spoonful of peanut butter about an hour before my morning ride, and that takes care of me quite well.