How much time does it take?

How do you judge the merits and demerits of the language?

Do people who know some PL theory judge it in a different way than those who don't? Like do they have a mathematical framework in mind and map the language docs to that framework to see what is bad or what is good?

For example:

Suppose I am to learn Python. I go to the official website and go to the docs page: https://docs.python.org/3/. I see a Tutorial section and a Language Reference section. I go through the tutorial. It is just a sneak peak. Then the Language reference?

Or do I read the Python source code?

P.S: For PL theory people what book/course do you recommend for that mental framework?