Here's why:
* It's not unusual: Dogs biting people is relatively common, and therefore not particularly newsworthy.
* Lack of novelty: It doesn't usually involve a unique story or unusual circumstances.
Therefore, there's no specific duration for "Dog Bites Man," as it's not a film or show, but rather a general idea.
If you're looking for a specific film or show with that title, it's likely you're mistaken, and it's a play on words referencing the common news trope.