XCOM 2: main story about 24 hours; story with some extras about 45 hours; 100% completion about 68 hours. These are IGDB player-submitted averages from 5 submissions, not a deadline or a prediction of your pace.
Source: IGDB player submissions
Community playtime data is not available for XCOM 2 yet. Tracked platforms include PlayStation.
Game Tales keeps what happened between you and a game in one Tale you can keep building.