Steins;Gate: main story about 18.5 hours; story with some extras about 20 hours; 100% completion about 40 hours. These are IGDB player-submitted averages from 3 submissions, not a deadline or a prediction of your pace.
Source: IGDB player submissions
Community playtime data is not available for Steins;Gate yet.
Game Tales keeps what happened between you and a game in one Tale you can keep building.