Prey: main story about 9.2 hours; story with some extras about 10 hours; 100% completion about 12 hours. These are IGDB player-submitted averages from 3 submissions, not a deadline or a prediction of your pace.
Source: IGDB player submissions
Community playtime data is not available for Prey yet.
Game Tales keeps what happened between you and a game in one Tale you can keep building.