One of my closest friends is a New Yorker, married to an English guy, and living in London with their two children. This is her second spell of living here, and after a combined total of over five years, she's as 'Noo Yawk' now as the day she arrived - not a trace of a dent in her accent. We speak on the phone two or three times a week, and I find my speech speeds up to match hers, and I start using some of her speech idioms, so if I moved to the U.S. I'd loose my British accent in about three days!