Raced through the game to get to the DLC endgame, took me 21 hours
I needed a second Hoopa for my living form dex so I sped through the game over the past week or so. Minimum requirements are to have main story 39 and DLC mission 12 completed.
The main story took 10 hours, getting Floette (mission 39) took another 3 hours, Darkrai took 5 hours, and Rayquaza (DLC 12) took 3 hours. I remember there being a lot of cutscenes in the story, but with minimal gameplay between them it basically becomes an A Button mashing simulator. Also, I did the Infinite Royale quest last just in case it wasn't necessary. Unsurprisingly, you do need to beat Urbain/Taunie to get the Hoopa quest.
This was pretty suboptimal, I'm sure it could be done much quicker. Especially if you transfer pokemon over from Home to steamroll through battles. I basically soloed everything with Bois until the DLC, where I got a Annihilape and Gourgeist to take care of rogue megas for faster mega dimension research.