If open world I agree that the main plot should be 15-20h long, to give even casual players a chance at closure. But for side content sky's the limit. If I actually vibe with your game I will likely spend months in it so stuff it all in. Witcher3 and Cyberpunk did this approach right (albeit more repetitive in cyberpunk). Besides, there's people spending thousands of hours in roguelikes, so 100h+ of non-roguelike non procedural content isn't that much in the great scheme.