That's great to hear. I personally think and always thought that way that a game should really have measured proportions in terms of collectibles and play time. So To me 4-5 hours is a pretty good experience. I think above 8 would feel too demanding for players.
Successful indie games like Journey and Abzu, Vane and others also manage to balance these aspects pretty well, and they usually don't take longer than 5-6 hours. So that's great to hear.