At least week and a half on the Big Island
I highly recommend at least a week or a week and a half on the Big Island because everything is so spread out.
From the resorts, it is a day for coffee farm tours and a couple days in Kailua Kona shopping and looking around.
A full day to visit the green and black sand beaches. Then a day for Waimea, Pololu and Waipio Valleys and maybe a trail ride to see the valley.
You can spend a day driving up to the visitors center on Mauna Kea and then down to Hilo and waterfalls at Kahuna/ Akaka Falls and over to Rainbo falls. Then anywhere from a few hours to all day at the rainforest zoo to see the animals and tigers.
A day trip to Volcanoes National Park is a great trip but to really see lava flowing it is down below the park and a few hours each way at night. Then that long drive back to the Kona side.
Oahu maybe 4 days, Kauai 3 or 4 days, etc.
You really have to decide what you want to see and what is important to you. Some islands have better beaches, some better shopping, some better photographic experiences.