Depends entirely on how hard I'm going.
Basically, your ability to metabolize fat goes down as lactate goes up, reaching zero at VO2max intensity. If it's a hard ride near or at threshold, you're not only expending more per hour, you're also metabolizing less fat, and consequently without eating, you start to fade and then at one point you're toast. If going easy on the other hand, then just the occasional bite every hour is fine.
For those who measure power, every 100W is 360 kcal / hr not counting in basic metabolic expenditure, so at 200W it is already 720 kcal/hr, and so on.