Half-Life: main story about 7.1 hours; story with some extras about 12 hours; 100% completion about 18.7 hours. These are IGDB player-submitted averages from 21 submissions, not a deadline or a prediction of your pace.
Source: IGDB player submissions
Community playtime data is not available for Half-Life yet.
Game Tales keeps what happened between you and a game in one Tale you can keep building.