There's a couple different stages and you can stop at pretty much any point: being able to read it, understanding all the ideas, writing with correct idioms, writing fluently, being able to design projects/systems effectively in it, and finally coding stability.
I know a lot of languages and learning new ones gets easier as you learn more of them. I can read pretty much anything aside from some APL variants. It usually takes me 2-3 hours to get the ideas for most. I can usually get to writing reasonably good code over a weekend and that's usually enough to fix bugs or contribute code following someone else's design patterns but it's fairly slow going. From there things slow down a lot because you need to work your way through the ecosystem learning the common libraries and their interfaces so it's usually 2-3 weeks full time before I really feel fluent and 6-8 weeks full time before I'd be confident in adopting it professionally. I find that my coding style stabilizes (I look at code I wrote 6 months ago and would still write it that same way) in a language after about a year and a half.
This timing varies a lot depending on the language. Languages that are designed to be familiar to C#/Java/JS devs like Dart or Go and you fall in that camp, you can blast through the tutorial to get the syntax and be into learning idioms. If you're significantly shifting paradigms (e.g. picking up your first functional, logic, or concatenative language) then you can expect to spend a couple weeks being blocked in the ideas phase. Switching domains is similar so if Rust is your first systems level langauge, you're going to spend a LOT of time on OS and memory details that other languages handle for you.
I know a lot of languages and learning new ones gets easier as you learn more of them. I can read pretty much anything aside from some APL variants. It usually takes me 2-3 hours to get the ideas for most. I can usually get to writing reasonably good code over a weekend and that's usually enough to fix bugs or contribute code following someone else's design patterns but it's fairly slow going. From there things slow down a lot because you need to work your way through the ecosystem learning the common libraries and their interfaces so it's usually 2-3 weeks full time before I really feel fluent and 6-8 weeks full time before I'd be confident in adopting it professionally. I find that my coding style stabilizes (I look at code I wrote 6 months ago and would still write it that same way) in a language after about a year and a half.
This timing varies a lot depending on the language. Languages that are designed to be familiar to C#/Java/JS devs like Dart or Go and you fall in that camp, you can blast through the tutorial to get the syntax and be into learning idioms. If you're significantly shifting paradigms (e.g. picking up your first functional, logic, or concatenative language) then you can expect to spend a couple weeks being blocked in the ideas phase. Switching domains is similar so if Rust is your first systems level langauge, you're going to spend a LOT of time on OS and memory details that other languages handle for you.