My first cruise was 6 days, 17 hours and 21 minutes.
Actually that is probably close.
I would never recommend anything less than a seven day cruise to anyone, especially if they have to fly in. Three, four or five day cruises are too short.
People think they will feel trapped but the ships are so large and there is so much to do I don't know how you would feel trapped.
I also recommend an outside or balcony cabin.