I have a new 20yr theory -
1954- Lord of the Rings was first published and gathered a worldwide following.
1977 - Star Wars, and we know what happened there.
1997 - Harry Potter. Same core mythology. Same characters. Fulfills the same need as the last two and becomes insanely popular.
Roughly 20yrs between each. Now, what do you suppose we can look forward to come c2017??