Can you build your fictional world too much? Not really, no.
The more you know about the culture, customs, geography, dialects, religion, mythology, technology, creatures, magic, and what-have-you of your world, the more you’ll be able to make that world three dimensional for your readers.
You can think of your world in three “dimensions”: Geography, Culture, and Conflict. Pretty much any facet of your world can be stuck under one of those.
Here’s a lovely graphic:
It’s important to develop in all three of these dimensions as you build your world. Also, the dimensions need to be balanced. Drawing a really neat map isn’t much good if it makes no sense in the context of the culture you built; coming up with a language for your main character’s society doesn’t help much if it’s incongruous with conflicts and changes in the society’s history.
Consider how the three dimensions affect each other. For example:
- How do waterways affect your main character’s society? (Geography -> Culture)
- How do barrier-type landforms, resource-rich regions, or climate affect current and past conflicts? (Geography -> Conflict)
- How do religious or cultural differences between groups affect borders? (Culture -> Geography)
- Do different versions of history create conflict between groups? (Culture -> Conflict)
- What battle-scars or ruins have been left on the face of your land? (Conflict -> Geography)
- What group does your main character’s society ridicule or fear because of past interactions? (Conflict -> Culture)
While we all like a well-developed world, character is almost always what keeps readers reading. Whether you build a character within an existing world or you build a world around an existing character, consider how the geography, culture, and conflict affects him or her.
You can’t do “too much” worldbuilding, but you definitely can do too little writing. Nobody’s going to care about your world unless you tell a story (even J. R. R. Tolkien had to tell stories to get people to care about Middle Earth). If you find yourself worldbuilding as a way to procrastinate writing actual words, stop. You’ve got enough to go on. Finish your first draft, and then go back to building the world. Revise accordingly.
In addition, while it never hurts for you to know more about your world, don’t bombard your readers with facts and histories that aren’t critical to the plot of your story. Again, nobody cares how such-and-such overthrew such-and-such unless there’s a story with engaging characters. Maybe you can write more books in the same world to tell those stories, maybe your readers will never know, or maybe someone will publish your notes Silmarillion-style after you’re dead. Either way, resist the urge to explain.