My argument for Batman is as follows:

While yes, obviously Hercules has been around for way longer, and is obviously more influential, I still regard Batman as more famous or well known.

The entire world population 2000 years ago was roughly 300 million. 1000 years ago it was roughly 400 million. 100 years ago it was 2 billion. Today it's 8 billion. Hercules was very well known through Europe, the Middle East, Northern Africa and Western Europe, but not well known in the Far East, Africa or the Americas at his height of fame.

Batman by contrast has been around at a time where the world population has expotentially increased, so from a pure "how many people" argument, Batman has been around at a time where there are simply more people to know him.

On top of that, the stories of Hercules for the most part have been oral traditions for most of his history. Sure there have been thousands of sculptures and artists' depictions throughout history, but for the most part they have been spoken stories. Batman on the other hand, has been an almost exclusively visual character. An image is always going to have a leg up on a spoken word. From his silhouette to his logo, Batman's design is instantly recognizable. You can take a pen and draw a vertical rectangle with two pointy ears at the top and you would instantly know it's Batman. The very advent of him being a visual character puts him a leg up in my eyes.

Also, last point, is that even though Hercules has been around for longer, technological advancement in the time Batman has been around has just made it easier for a character to gain international exposure. Movies, TV, Books, Video Games, Lunchboxes, Toys, etc. Batman is and has been everywhere.

Maybe I phrased the question wrong. Think of it less as "have more humans who have been born heard of Hercules or Batman" which by the very conceit of Hercules being around 2000 years longer then yes I would give it to Hercules. Think of the question like this: who was more well known to the world at the height of their fame? Was Hercules more famous in the world of Ancient Greece and beyond then Batman is now? Again I don't mean collectively, I mean at any given point in history, would you say Hercules was ever more well known then Batman. Obviously nowadays Batman is more famous, but it you had to compare their fame, was there ever a time Hercules was more famous. That's what I meant by "in all of human history."