Jeff Gardiner, lead producer on upcoming Fallout 3 DLC Point Lookout, has told Platform Nation that the content will last for "several hours". You'd hope so, really.
Gardiner said Lookout wouldn't be "substantive" specifically "in terms of new gameplay features such as additional levels," but, "there are however lots of new creatures, items, armor, weapons and a whole new area to explore.
"The Point Lookout Swamp is substantive and should engross players for several hours even if they ignore the quest line."
There's an interview through there. The DLC's out tomorrow for 360 and PC.