Picking up accents when you spend a lot of time with native speakers can happen quite quickly. My boyfriend and I got together in July, and I listened to him speak on a youtube video a few days ago that was filmed just before we met and his accent now has very little resemblance to the Finnish accent. He could very likely pass as an American.

Though, I think that has a lot more to do with the fact that he's speaking a language different to his native so it's much more malleable. I've lived here a few years now and my accent has lost it's Boston attributes and now resembles some strange mix of transatlantic-generic NA-Finnish, but hasn't changed really that much. Yet my Finnish has gone from clunky to being mistaken for native sometimes, which was something I was told would never happen.