Reddit mentions of Jocks and Burnouts: Social Categories and Identity in the High School

I'll begin by stating that I studied sociolinguistics in grad school, though it was a while ago so I'm not up on the latest research.

However, that said, there is NO SUCH THING as 'accentless'. Everyone has an accent. There's an interesting book called Language Myths that I think you'll really enjoy. It's written by linguists (which, I'm sorry to say, Bill Bryson is not) but doesn't indulge too much in jargon. It will help dispel many of the myths non-linguists have about language (English is harder to learn than other languages, it has crazy ass spelling, German is crazy, etc.). Also this Intro to Sociolinguistics book available on Amazon is pretty easy to understand without any training. And although to an untrained ear everyone who lives in the same city and even attends the same high school will sound the same, linguists such as Penelope Eckert have shown significant differences in speech traced back to social group. Here's a chapter of the book if you just want a taster. So there's probably a lot more diversity in your city than you think!

I can also speak about the second part of your question, as I'm an American living in the UK. Been here over a decade. There IS more dialect diversity in a smaller area than, say, Canada or the US. However, there is no finding native speakers of the next town over incomprehensible. There are accents which can be quite thick and difficult for outsiders (say, South of England) to 100% comprehend (Aberdeen, I'm looking at you!). However, it is NOT the case that a Glaswegian would struggle to understand someone from Aberdeen. Someone from York will not struggle to understand someone from Southampton, and even I struggle to see much difference between native Yorkers are native Scarborough residents.

How long does a dialect take to arise? That depends on a ton of other stuff so there's no easy answer to that. The Northern Cities Vowel Shift happened relatively quickly. And you've probably heard it on telly and didn't even realise it. In Spongebob, Plankton's computer wife Karen has a very strong NCVS accent. She doesn't say 'that', she says 'thee-at'.

I have really shitty hearing these days so I don't do any linguistics stuff. However, the amazing folks over at /r/linguistics would be happy to help explain dialect development further, and frankly they're more up to date than I am.

Edited for clarity and to add more links and whatnot.

There is a lot of interesting work out there on dialects. I really like these 3 studies -- please note that they're pretty much in the realm of "classics" and there is MUCH newer work, but these studies (and many others) are considered foundational.

• Walt Wolfram (1969). A Sociolinguistic Description of Detroit Negro Speech.
  
• William Labov (1972). The social stratification of r in New York City department stores
  
• Penny Eckert (1989). Jocks and Burnouts
  
  This one's new and I haven't had a chance to read it, but I've heard good things. You might like the book "Roots of English: Exploring the History of Dialects" by Sali Tagliamonte.