The beauty of D&D 4th Edition is that it's more geared towards these "Done in One" adventures. It's been restructured to be mor elike a cross of a Television Show and an MMORPG instead of the traditional model of sweepingly epic adventures like Lord Of The Rings. Oh, you can still string your adventures together to form a ginat epic, but it'll be an episodic epic.

An hour or two is the most you need now :)