It depends how many "main attractions" you plan on doing. My fiance and I are honeymooning in Hawaii for almost 2 weeks. The first part is a week on Maui and doing absolutely nothing but hanging out by the pool and the beach because it'll be right after the wedding. The second week will be spent on Oahu and staying at Aulani. I'm a little crazy with the planning and came up with a whole itinerary to do on Oahu, with every other day spent at the pool. We're going to do an all day tour of Oahu, go to Pearl Harbor, the Kualoa Ranch, the Dole Plantation, and a luau. Some of the things we doubled up on, going to more than one site in a day. It was also a little easier for me to plan since I did this exact trip back in 2012 after I had won a trip to Maui when I was on Wheel of Fortune.
It really is up to you how much you want to do or not do. Either way, Hawaii is beautiful and you'll want to go back again so anything you don't do, you can hopefully do on another trip.