While some game's DLC packs can be played in one sitting, The Witcher 3's two expansions will be far more substantial. CD Projekt RED's Konrad Tomaszkiewicz said in a new interview that the DLCs might provide as much gameplay as Witcher 2.
Tomaszkiewicz told GameReactor that the two expansions provide a combined 30 hours or so:
The expansion packs will be whole new storylines with all the elements you've got in the main story (like cutscenes, new characters, new items, new Gwent cards) and many, many more. They will be quite long. The first expansion will be around 10 hours and the second expansion will be around 20 hours... and I think it's possible that they will be bigger, because it's always like this (that when we plan some time or hours, it's twice or something like this), but I'm not promising anything right now.
The first expansion pack is called Hearts of Stone. It's said to feature quests throughout Velen and the city of Oxenfurt, locations players visited in the main game. Geralt is given a contract by a shadowy figure known as the Man of Glass. Given Hearts' 10-hour length, I'm guessing that the mission turns out a lot more complicated than expected.
Blood and Wine, the second expansion, brings players to a brand-new region called Toussaint. Toussaint is a mountainous area in the Nilfgaard Empire. While it hasn't been affected by the war raging through other parts of the world, Toussaint has its own dark secret for Geralt to uncover.
The catch is that players are going to have to wait awhile for these expansions. Hearts of Stone is slated for October while Blood and Wine won't be out until 2016.
In the meantime, CD Projekt has been releasing smaller DLC packs for free each week. These add-ons have introduced new armor sets, alternate costumes for characters, and quests. While the quests are fairly short, they're interesting experiences and worth checking out.
Even without DLC, The Witcher 3 is massive. I've played about 80 hours thus far and I still have side quests to tackle. I haven't even touched that Gwent minigame, either.