While I agree with everything that cblaisd and TheMightySkunk wrote (very informative - thank you), I can pretty much tell you that unless you have a *really* mellow island disposition, a month on Moloka'i is too long.
I spent a week there, and while it was longish, it was just right. At the end of the week, I was ready to leave. (Though I would go back...)
P.S. It's one of the places where I've come real close to drowning, so I do have a healthy respect for it in a weird way too...