Cuphead: main story about 8 hours; story with some extras about 17.6 hours; 100% completion about 21 hours. These are IGDB player-submitted averages from 6 submissions, not a deadline or a prediction of your pace.
Source: IGDB player submissions
Community playtime data is not available for Cuphead yet.
Game Tales keeps what happened between you and a game in one Tale you can keep building.