I have never heard an actual number put to it, but it is a common statement that hybrids are better for shorter rides. The primary basis for this is that flat bars don't have multiple hand positions like a bike with a drop bar, so it might be the case that you are more likely to get fatigued in you arms and hands and shoulders.
In reality, people who ride long distances or tours (from my experience) are on hybrids or other flat-bar bikes half the time or more. The only situation where flat bars are universally undesirable is in mass-start road racing. But otherwise people ride every distance you can imagine with hybrids. If you've got a hybrid and want to ride a long distance, the only thing stopping you is you.