Hmmm. Well, mine hasn't changed in 6 years.
I honestly think it depends. Some people morph without realising it and some people never morph. I know of a few folks from various countries whose accent has never changed and others who it becomes different.
Even my Dad, grew up in the southern US, but has lived 'up North' for more than 1/2 his life. So he sounds quite generic (I don't have a Massachusetts accent either!) except for when he gets around family or friends from Maryland/Virginia/West Virginia/Georgina/Northern Florida - he morphs and doesn't even realise it.
Likewise, I know people who have lived in California for 20 years but grew up in Massachusetts and have never lost their Massachusetts accents....
Meanwhile, today on the phone with someone I confused the heck out of someone who was like, 'Your accent is throwing me off. You sound Scottish, but not quite. I'm very confused!' I had to giggle.
So you my dear, are probably someone who is just not prone to these things. Go you, never change your beautiful self!