I have seen on various sites listing an estimate of how long it will take to read the article.
I do not expect these sites have measured how long it actually takes an average read the article. Instead, I assume they use a heuristic based on average reading speed in words per minute and then just count the words in the linked article.
An example of this is https://medium.com/ as can be seen here:
How can I make Jekyll estimate the time to read an article/post?