Just a few hours. It does have a ton of replay value, but I'm trying to wait for the full game. I don't want to play the demo so much that it kills my buzz or gets monotonous.
But sometimes, man, I just get the urge to kick some demo arse, and Bloody Palace in the old DMCs isn't quite doing it for me anymore with the new gameplay available. It's just hard to get quite as into the old games as I used to, although I did come up with some weird challenges for myself (kill Vergil with Gunslinger and the Shotgun alone).
I wonder if there's any weird constraint I can give myself for the demo to make it more frustrating.