Portal 2: main story about 4.1 hours; story with some extras about 10.4 hours; 100% completion about 27.1 hours. These are IGDB player-submitted averages from 35 submissions, not a deadline or a prediction of your pace.
Source: IGDB player submissions
Community playtime data is not available for Portal 2 yet.
Game Tales keeps what happened between you and a game in one Tale you can keep building.