Reply #2: A couple hundred years, probably, to be considered different languages

[ZombieNixon]

a la English vs. Scots. A couple more hundred to be mutuall unintelligible, like English vs. Dutch.

Hell, if public schooling went the way of the dodo and only rich people could afford a full education, you could even begin to see a more significant diversion in the registers of the upper vs. the working class in a single area, approaching very different dialects (compare RP English with a dialect like Northumbrian).