heya!
take it from someone who's been thru the game 4 times, and partway with about 5 or 6 other teams...
Ole Dark and Grumpy shows up whenever he wants. ususally within an hour gametime when you hop into the swamps. but sometimes as much as a few days gametime!
and it seems that being near a "swamp gas" spot will encourage him to come around. but it should be one that's close to one of the paths that wind around the swamp area.
don't worry, he WILL turn up. i don't think there's ever been any reports of a complete no-show for him.
V***V