There are 4 temples to defeat once you become a collector and then there will probably be some kind of final boss after that so maybe the demo covers a 6th of the game in terms of story content. But the bosses get harder and harder so you will spend more and more time retrying bosses the further you get so the story will be longer than 6 hours at least. Then you have all the side quests to complete, cards to gather, fishing and items to find so with that you maybe have minimum 40 hours without playing Arcade Mode.
Right now you have about half the story to play through so the current build of the game would last maybe 20-25 hours on a Normal playthrough. I'm pretty much guessing numbers here but I know that most of the active members here on the forum have over 40 hours in the game.
Hope this helps.