Whatever I feel like on that particular day, during that particular ride. I've done 100 miles on a Clif bar and a pack of mini donuts, but I've also eaten 3 full meals during a double metric. I've done 100km on nothing but water, and done 100km while eating a full meal in the middle. Don't get hung up on it. If you feel hungry, eat.
Obviously, YMMV. I average 45kJ/mi, so a century for me is going to be good for no less than 4,500 calories burned. I'm not worried about my figure when doing 100 miles.
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