How long does it take to lose a foreign accent?

There’s this female professor I know who’s been here in the U.S. for over 25 years, and yet she still speaks with a HEAVY Austrian/ German accent. I’m wondering why that is, since she has no family here aside from her husband who is American Irish. I would’ve thought all those years of living here and teaching (the student population is pretty standard – mix of local and international students, the latter mainly from India, China and Korea).