I'm with you on this one - my wife's brother went to uni in London. His Yorkshire accent gradually dwindled and although there's traces there, he's started using the long 'a' sound in words like grass and bath.
One of her cousins went over to Tasmania after her post doc and has got a heavy Australian accent now with hints of British coming through. She's been there for around 12 years.
Conversely we've got Londoners at work here who've not lost their accents, so the whole thing must come down to who/what you're around, like you say, and the susceptibility of the individual to those outside influences. Also if you're proud of your accent you'll probably be more conscious of trying to preserve it, likewise, if you come from Romford (and have any wits about you) you'll be trying your best to disguise it.
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