Sonic CD: main story about 1 hours; story with some extras about 1.9 hours; 100% completion about 2 hours. These are IGDB player-submitted averages from 2 submissions, not a deadline or a prediction of your pace.
Source: IGDB player submissions
Community playtime data is not available for Sonic CD yet.
Game Tales keeps what happened between you and a game in one Tale you can keep building.