I want to make it clear from the start that I'm not any kind of expert in linguistics or history, which is why I'm asking this question here. (Perhaps it's because of my physics background that I'm wondering about "characteristic timescales.")
I've heard that Polynesian languages, which spread throughout the Pacific Ocean starting 3000 years ago, are very similar to one another. In Wikipedia, for instance, it says
Still today, Polynesian languages show strong similarity, particularly cognate words in their vocabulary; this includes culturally important words such as tapu, ariki, motu, fenua, kava, and tapa as well as *sawaiki, the mythical homeland for some of the cultures.
Meanwhile, (pre-European) languages in North and South America are described as being extremely diverse. From ayahuasca.com,
Now consider the following languages: Salinan, Shasta, Yuki, Maidu, Pomo, Yokuts, Esselen, Washo, Karuk, Chimariko. All are from present-day California, but what is notable about these languages is that each one of them represents a unique isolate, like Basque. That is ten isolates in California alone — each utterly unique, representing an entirely unique language stock all by itself, and each as different from any other language as English is from Arabic or Vietnamese is from Zulu.
People first spread across the Americas at least 13,000 years ago (pre-Clovis discoveries could push that back farther).
I was wondering if these two cases of people spreading across large distances could be taken as a way to measure the rate of language drift: 3000 years is too short to develop into fundamentally different languages ("isolates"), but 13,000 years is long enough. So the characteristic timescale is somewhere between them, like ~10,000 years. (The tilde means "more than 1000, less than 100,000.") I'd like to know if this is something that has been written about, possibly rejected, that I could go read up on.
(In preparing this question, I came across a point that could weaken it: the original migrations into the Americas could have involved more than one language group, but it still seems hard for me to believe that the number of isolates in modern times came from that many independent migrations into the continent. Also, I know that human situations are different in different times and places, unlike drifting particles, but it would be nice to know if something universal can be said about this. Like, each generation is willing to change the language a certain amount, but not much more or much less, and if there are cultural variations, they average out in the large sweep of time.)