I don't have the answer, but I would assume not that far. As long as a save is recognised from the original, that is all that matters. So one would guess a save from the first possible point at the start of the game would be enough. Right?
I hope so, because I was planning on buying a copy of the original a few days before Dark Arisen, and then giving it to my nephew when DA arrives, nabbing the bonuses along the way. :P
I think the bonuses should be available the moment you open your vanilla save in DA. That is the problem, tho, you'll only get bonuses per save so you'll probably need several alts if you wanted the bonuses per game.
I played about an hour or two tops on the first game before I got sidetracked royally and ended up selling the game because I didn't ever feel like picking it back up. This new release will get me to buy and play it again, though. New content does that. I'm a bit weird that way, I guess, since I know the core game is pretty much unchanged.