I really, really don't like DLC unless it's obviously some sort of afterthought.
DLC should be exactly that - An afterthought, something neat the devs came up with that they didn't during development. As of now it seems the devs can say "Oh, don't worry if it's not finished - we'll release the game and a few months later we'll patch/fix/update the game with that thing, forget it!"
If you need 3-6 weeks to develop it - then put that in your initial timeframe.
DLC should be exactly that - An afterthought, something neat the devs came up with that they didn't during development. As of now it seems the devs can say "Oh, don't worry if it's not finished - we'll release the game and a few months later we'll patch/fix/update the game with that thing, forget it!"
If you need 3-6 weeks to develop it - then put that in your initial timeframe.