Sounds something like this.
That is from The Canterbury Tales. They were written around 1390, which is about 620 years ago. I do not know about you guys, but my intelligiblity score of Middle English was 5%. I think there might be around 100 words in that sample, not sure. Middle English is quite simply not the same language as Modern English. It’s a different language altogether.
So if languages are split for 600-650 years, they may only have 5% intelligibility. That is if they do not continue to have connections with each other. If they continue to have linguistic connections with each other via speaking together and living in the same vicinity as the other tongue, the score can be a lot higher.
For instance, Scots separated from English ~500 years ago but I can get a lot more of Scots than I can of Chaucer. My intelligibility of Modern Scots is ~40%. But you see, Scots and English continued to be in regular contact. If Scots had taken off to Sweden or someplace like that, the score might be a lot lower. Scots’ continued interaction with English slows the rate of differentiation between tongues.
So after 500-650 years linguistic separation, you should have separate languages, and intelligibility may only be 5-40% (average 22%).
