Whatever time frames you apply, the only way to really learn a language is to use it. Reading a book, blog articles, and watching videos on YouTube won't give you the hands on experience necessary to internalize how everything works.
Exactly. There usually is some stuff about the deeper, darker corners of a language in the type of book I pick.
I usually pick the authoritative book / advanced book.
Think "Programming PHP", "The joy of clojure", "The C Programming Language", etc.
I don't necessarily need to know all about performance optimization, the async model or non-html templating.
Reading a technical book cover to cover feels wasteful imo