Portal: main story about 1.9 hours; story with some extras about 4.1 hours; 100% completion about 7.4 hours. These are IGDB player-submitted averages from 48 submissions, not a deadline or a prediction of your pace.
Source: IGDB player submissions
Community playtime data is not available for Portal yet.
Game Tales keeps what happened between you and a game in one Tale you can keep building.