Firewatch: main story about 4 hours; story with some extras about 4.7 hours; 100% completion about 5.8 hours. These are IGDB player-submitted averages from 27 submissions, not a deadline or a prediction of your pace.
Source: IGDB player submissions
Community playtime data is not available for Firewatch yet. Tracked platforms include PlayStation.
Game Tales keeps what happened between you and a game in one Tale you can keep building.