If you are US citizen you are subject to US income taxation wherever you live. No matter how long you live there. Even green card holders are subject to the same rules as citizens.
This year I'm an American citizen living in FL. My lease is up in May and I expect to spend significant time traveling. Probably mostly Canada this summer. I am considering moving to Puerto Rico though in December. With their rules you don't pay any capital gains, but you're not allowed to have any other tax residency. I would like to spend significant time abroad as it is easy for me to be mobile.