Western education has been developing for the last hundred or so years. It has never been "finished". It changes and adjusts to accommodate the results which are the graduates and how they turn out. As history has shown, the results haven't been all that favorable so education in the West now more than ever is being revamped, redesigned, revived, etc. so that this society doesn't fall too far behind in the World's standings.
So, how long does it take to create a curriculum? The answer is multi-faceted. For one, a true curriculum is never finished because as time continues, changes take place in the universe. This creates a need for a curriculum to be organic and for this self same curriculum to be able to accommodate changes without losing its overall format. Next, only a limited minded being would think that a curriculum can truly be completed. This would mean that the process, methods and means of this curriculum are absolute. This would also mean that this curriculum would ultimately have to ignore changes that happen around it. Who wants to study from a manual that is twenty, forty, sixty or even one hundred years old, that has never been updated? Me neither...
The true question really then is how long does it take to create a template for a curriculum that can groom our children so that the best of their genetics are developed?
How about...six years?