Half-Life 2: main story about 8.8 hours; story with some extras about 13.5 hours; 100% completion about 20.4 hours. These are IGDB player-submitted averages from 25 submissions, not a deadline or a prediction of your pace.
Source: IGDB player submissions
Community playtime data is not available for Half-Life 2 yet.
Game Tales keeps what happened between you and a game in one Tale you can keep building.