To finish all G-Rank quests and/or to get all armor sets and weapons you will most likely sink close to 1000hours into the latest western release (Monster Hunter 3 ultimate) I have already played 120hours so far and I am at the start of High rank in solo and haven't touched multiplayer at all for now.
And if we ever get a western MMO like the Japanese have with MHFrontier or the chinese soon with MHO it would go into the thousands.
MH ist basically a MMO for offline and optional multiplayer part. So the transition between MH series and the MMO is barely noticable (you just see more people in town), gameplay is the same.
Dark Souls I have played twice through, once on the original PS3 version and once on the PC (prepare to die edition), MH just drives me froward because it actually rewards you with new enemies (even if its just subspecies) that actually get new moves to learn. In Dark Souls they just scale so they have more HP deal more dmg etc. Of course Dark Souls best part is the PvP which MH doesn't have. But for PvE MH is king compared to DS.
Dont get me wrong though, both games are great and I am awaiting Dark Souls 2 as much as I await any MH game to come, but since I like PvE more I enjoy MH a lot more.

These both games are better, more rewarding and skillful to play than any other new "CoD" that gets to us. Asian gamers have a lot better games than we do now. I wonder why the western gamers got so brainwashed into liking CoD and easymode games when the asians basically stayed the same at the core, hard games, grindy games etc.