We've spent a week on oahu, kauai, and big island. We didn't have enough time to see everything we wanted (and have lazy beach days too). 10 days to 2 weeks per island is better, especially big island. You really need to spend at least a few days on each side of that island otherwise you're driving too much. We're going 4 weeks next year, 2 on maui and 2 on oahu.