Subnautica: main story about 26 hours; story with some extras about 50.8 hours; 100% completion about 48.9 hours. These are IGDB player-submitted averages from 9 submissions, not a deadline or a prediction of your pace.
Source: IGDB player submissions
Community playtime data is not available for Subnautica yet.
Game Tales keeps what happened between you and a game in one Tale you can keep building.