I suppose it MIGHT be possible to clear it in 4-5 hours or even less, if you only do what's necessary for completion and Achievements, and have some decent skills besides. But there's other stuff to do, most noteably the Weapon Trials - you only need to clear 7 to get the related Achievement, but there's, what, 11 in all? 12? Something like that. And some of 'em are quite fiercely hard, depending on how good your sense of timing and reaction-speed is.
Between doing those and a little bit of XP-and-money grinding, MY first run came in at maybe 7 hours... and I still had to give up on a couple of the Weapon Trials. So assuming you just rush through your second run, I guess the final tally would be somewhere in the neighborhood of 11-12 hours, which is quite a respectable length for an XBLA-game.
You could easily stretch it longer if you want a challenge, though - the difficulty is highly adjustable. At its basis, it's quite easy, and you're unlikely to ever need to restart a level... but you can activate a number of 'idols' at the Temple (once you get it), each of which grants the enemies a certain advantage, such as increased attack-power, enhanced speed, health-regeneration, and others. Switching on a few of those will create a MUCH greater challenge, forcing you to really work for your progress, and lengthening the game significantly. Activating the 'Increase Defense', 'Regeneration', 'Random blocking' and 'Random untouchable' idols will turn any boss into a difficult Marathon-session...
77777 reached - 17/11-2011
Next goal: 99999!