I'm an average weight person trying to lose 5-10 pounds, but possibly like a lot of people, I tend to use the fact that I exercise as a good excuse to eat more. "Oh, I'll need the extra energy later today or tomorrow when I run." That makes sense if you're about to run a race and need top performance out of your run. But when you're trying to lose weight, there must be a balance point between not consuming more (so you'll burn more than you take in) and eating enough to avoid passing out halfway into your workout.
So, assuming a person of say, 35, with an average weight and in reasonably good health is running a moderate pace on a flat surface, where, on average, is that point at which s/he needs to increase intake? I would take the answer in either distance or duration.