How long do the first 3 books of Korra take?

While discussing Bolin's loyalties in Book 4 of The Legend of Korra, a friend of mine made the point that Bolin may have, at that point, spent more time working with Kuvira than he had spent with Korra. I don't think we'll ever be able to say for sure, since we don't know exactly when he joined up with the Great Uniter (all he says in his letter to Korra is that he got a job working for Varrick, and thus presumably joined Kuvira whenever Varrick did).

But do we know how long the first 3 books of Korra took, all told? As I recall, Book 3 started shortly after Book 2 ended, but I can't remember the span between 1-2, or how long the books themselves were supposed to have taken.

By the ceremony scene at the end of Book 3 of The Legend of Korra, how long had it been since Korra left the South Pole during the pilot?