Little Nightmares: main story about 2 hours; story with some extras about 3.6 hours; 100% completion about 9 hours. These are IGDB player-submitted averages from 13 submissions, not a deadline or a prediction of your pace.
Source: IGDB player submissions
Community playtime data is not available for Little Nightmares yet. Tracked platforms include PlayStation.
Game Tales keeps what happened between you and a game in one Tale you can keep building.