Setting: Neat-future fantasy world. Technically, all of these are fantasy countries that resemble real-world countries in certain ways, but instead of writing about "an invented country with an accent I represent as an English Accent" or "an invented region that speaks a dialect similar to Southern American English" or "a city which has certain sociopolitical and geographic similarities to London" or whatever, I'm just going to use the actual names since the differences aren't really important here.
The situation: I have a 30 year old character who was raised speaking American Southern English (let's just call it Coastal Southern), and has lived in the South for most of her life (except for a few years here and there spent abroad in Japan). Her parents are immigrants from India who speak (fluently) with Indian accents, but that never really rubbed off on her.
Now she's a postgraduate student working on an astronomy PhD at a highly prestigious university in London. The people she interacts with on a daily basis are all either highly educated university types from various locations around the UK (mostly Southern England, but probably a mix of everywhere from Scotland to Wales to Yorkshire to wherever), or other international students/faculty who were taught British English. (She also knows a number of people from Japan she speaks to in Japanese but that's not really relevant) The only Americans she talks to on any regular basis at this point are her parents when she occasionally calls home amidst a tidal wave of postgraduate work.
Given this, I have three questions:
1.) I know that the rate at which accents are dropped and picked up varies a lot, but what's the fastest she could lose her American accent without her consciously deciding to drop it. She's not making any particular effort to keep the accent, but she never sits down and thinks, "Wow, time to stop sounding so American!" specifically.
2.) Given 1.), what would she sound like after about six months in London? Would her accent have changed enough that another American would notice it?
3.) What sort of English accent would she emerge from all of this with? Would it be a London accent? Would it all just iron out into Received Pronunciation? Would it just sound generically English to Americans and be sort of unidentifiable to UK natives?
Possibly relevant factors:
1.) For her whole life, a big portion of the English-language media she's been exposed to has been British. This is actually because of various geopolitical factors in the setting, but for the sake of argument let's just say that she's a BBC-loving Anglophile, since the net effect-- she's used to hearing British accents on TV and in the movies-- is the same.
2.) She's fluent in Japanese and knows her way around Latin, so she's no stranger to thinking about language in terms of pronunciation and accents.
wow that was kind of a long question sorry ;_;