I've been to Kauai four times. My visits have ranged from ten days to three weeks. Kauai is a great place just to be. As others have said, if you want cities and nightlife, don't go. If you want to relax, smell the flowers, be outdoors and be inspired by great beauty - then spend as much time as you can spare.

If I have a week or less, I would stay on the east side, because it is easy to access both the north and south sides from there. If I were staying longer, I would consider splitting my stays between the south and north sides.