For those concerned without spoilers, Aside from the shared Prologue/First 5 Chapters, the games are full fleshed out. Will not comment on how much as it will make some angry here.
As for the third Path DLC, the obvious main character decides not to join either side and runs. Its a game practically in itself. Only thing i can say is in Main game its as long as Awakening (this includes ch 1-5 which are the easy ones, duh)
Hopefully this clears up some air to the separate versions. 3 FE games practically, DAY 1! :0
Spoilers on third path Chapters (or just to see how many) Click Link http://serenesforest.net/forums/index.php?showtopic=54925