Oxenfree: main story about 3.8 hours; story with some extras about 5.2 hours; 100% completion about 10.3 hours. These are IGDB player-submitted averages from 7 submissions, not a deadline or a prediction of your pace.
Source: IGDB player submissions
Community playtime data is not available for Oxenfree yet.
Game Tales keeps what happened between you and a game in one Tale you can keep building.