After struggling several times for hours with the Lightbaking System in Unity5, I finally got to this post, which says that there’s a bug in Unity where the Lightbaking Process get’s stuck (or at least takes up way too much time) at some indirect light calculations, but only when the Precomputed Realtime GI’s Realtime Resolution is greater than 0.2

So watch out and keep the value lower than 0.2 !