I think you'd be surprised. It took less than a year and a half in Australia for my already confused accent to start changing. Nearly a year after I got back to the UK, people still thought I was an Australian. Which was annoying, as I thought I was sounding more and more British the longer I stayed in Oz.
Mind you, I met a couple of ten-pound tourists in Vancouver who had been there for nearly 40 years but who still sounded like they'd just popped over from Liverpool and Essex. The poor sods.
I've got the accent sponge thing a bit too. I've tried very hard to resist picking up too much southern though. I'm not going to start saying grarse or barth or anything, ew.
It's most embarrassing talking to foreigners and adopting their slightly broken English. What a pisstake!
I think this might be the most famous sufferer - former England football manager goes to work in the Netherlands and adopts comedy Dutch accent for interviews.
http://www.youtube.com/watch?v=xhtq1ObGHy8