Learning a programming language gives you the opportunity to explore other problem domains alongside new approaches to addressing familiar problems.
I write this as I was looking for the answer to exactly this question. There were many opinions that had me confused, some from ancient rockstar ninjas who command CPUS at will and others from earnest people who are also seeking.
Why would one ask such a question in the first place? It seems like the actual issue is 2 fold. For me it is firstly; when can this go on my resume. Secondly and probably more specific to me: when to move on to the next language or skill. When can I check off my to-do list and say that I know a programming language?
After looking at many other ideas, In my opinion: You are proficient in a language when you are able to read, understand and debug code along with deploying to production.
If you can read, write and execute you are well on your way and I would consider this profecient. Beyond this, optionally, persue mastery. Though in order to keep a language on your resume, make sure to continually read, understand and debug.