Length of this game depends solely on how you play it, the game is quite easy and you can beat it really quick even if your characters are on low levels because opponents level simultaneously with you. Doing sidequest's and collecting cards add some serious length to this game.

As I said before, this game is really easy but only if you understand how to use junction system and you obtain most important magic to junction, abilities of GF-s, also known as summon creatures are most useful especially those that raise your strength and HP. Ultimate weapon's can prove to be also useful because of devastating limit breaks they enable such as Lionhart. You can also use most of opponents weakens to your advantage via junction system enabling you to defeat some strong monsters in one blow like Behemoth for instance. Anyway, I don't think you should have any problems beating this game and I'm sura you'll enjoy playing it.