Let's assume Britain, England. At what point would it be difficult to communicate but possible, and at what point would we simply be too different to communicate in any verbal way.
There are a few questions similar to yours in the past that you may find interesting:
There are some answers Here in the FAQ that might interest you, particularly the one by /u/bloodswan
I also wrote a similar work here
I've found some older answers to the question as well, but a lot of them are from older days of this forum when standards were less strict, and I don't fully agree with their conclusions, though I do feel like I've read better answers to this exact question than I was able to find in a cursory search. Essentially, the summation is this, though: 300-400 years would involve only minor difficulty. 500 years is probably a fairly hard limit for easy communication, but even 600 years ago, at least in London and the surrounding area, you'd probably be able to at least make yourself understood with some effort, and within a couple of months would probably have little difficulty (I was able to stumble through Middle English writing pretty well within a few months of study, and immersion would be a far quicker teacher). Happy to answer any followups you might have!