https://www.gamespot.com/articles/horizon-zero-dawn-the-frozen-wilds-campaign-length/1100-6454698/

Horizon: Zero Dawn's first DLC, The Frozen Wilds, is just one day away on PS4. With it comes a new story, new quest lines, errands, weapons, armor, and a new skill tree (which will be available in the main game too). GameSpot has played through The Frozen Wilds and, while much of the DLC's content can have far-reaching effects on the game as a whole, we can tell you roughly how much play time you'll get out of the new features.

We got through The Frozen Wilds in about 12 hours, but the DLC could take you anywhere from 10-20 hours depending on your starting level--the game recommends you be level 30 before attempting The Frozen Wilds). In the DLC, you'll explore the icy world of the Cut and experience the game's new weather effects; wind and snow particles affect how well you'll see.