L.A. Noire: main story about 15 hours; story with some extras about 28.7 hours; 100% completion about 42.1 hours. These are IGDB player-submitted averages from 14 submissions, not a deadline or a prediction of your pace.
Source: IGDB player submissions
Community playtime data is not available for L.A. Noire yet.
Game Tales keeps what happened between you and a game in one Tale you can keep building.