Hollow Knight: Silksong: main story about 17.1 hours; story with some extras about 51.5 hours; 100% completion about 82 hours. These are IGDB player-submitted averages from 27 submissions, not a deadline or a prediction of your pace.
Source: IGDB player submissions
Community playtime data is not available for Hollow Knight: Silksong yet.
Game Tales keeps what happened between you and a game in one Tale you can keep building.
Start your Hollow Knight: Silksong Tale.