From what I understand about Darksiders you're looking at 15 - 20 on normal difficulty without looking for the other items or exploring new areas. This sin't a linear game like God of War. Think Zelda, Metroid, etc. Once you get a new weapon or item you go back to a previous spot, use that item or weapon or technique and open up new areas.
With FF you're looking at 40+ at least. I mean it's FF, bro.
As far as Dante is concerned, that is a linear game in the vein of God of War, so I wouldn't expect the campaing to be longer then 10 or 12 hours max.