This kind of response is on the more uncharitable side of the responding to OP's intention.
If it takes 'decades', you might as well give up now, if you're considering a job opportunity that uses a different language than the one you're used to.
"A lifetime" is not a helpful response in what an honest question like this, because usually everyone realizes that full mastery on the level of compiler implementer is not what is implied by such a question .
Did you miss the part "to a level where you can start contributing to a project with it?" In OPs question?
If you grow with an eco-system from the day that it starts to be marketed publicly to where it is today you have seen all the good, bad and ugly that came with it and you deeply understand why things are done the way they are done. If you are dropped into an already existing eco-system (say Java) it will take you a very long time to catch up with what for old hands is 'obvious'. Maybe I'm a slow learner, maybe I lack confidence, but for me before I'm ready to go and 'show my work' it takes a long time.
Even now people are asking me almost on a daily basis to post a bunch of python code that I wrote and I'm probably not going to do it even though I fully intended to because - to me - it looks like crap, and I lack the time to make it better.
A couple of extra notes: I'm 'old school', never found an IDE that I liked and tend to memorize everything. This takes a long time. A typical library is a couple of tens to 100's of functions and getting to know such a library (or API, for that matter) takes time as well. Which makes me totally unsuited for certain kinds of programming, the kind where the eco-system moves faster than I can keep up with it.
Did you miss the part "to a level where you can start contributing to a project with it?" In OPs question?
I think you are the one being incredibly uncharitable. Here is the question:
"how long do you find it takes you to learn"
This is his answer for how long it takes him to feel he has adequate mastery. Perhaps he should have refrained from using the word "you" a few times, but it is not common in English to say "one," so you is often used to mean "people generally" rather than "You in specific, the person I am currently addressing."
It often causes problems in online communication and I have had my head handed to me over using the generic you on quite a few occasions, which is an incredibly uncharitable reading that usually gives me the impression the person was basically looking for an excuse to attack me personally for some reason. (This was years ago on a list where that kind of nonsense was quite common. I don't specifically recall it happening to me on HN, in part because I have worked at avoiding the generic you in any instance where someone might use it as an excuse to rake me over the coals).
You can spend the rest of the day wondering (privately, to yourself) if he is merely neurotic or if he has some kind of learning disability or if he has wisdom of experience that perhaps others should learn from (or all of the above -- none of them precludes the others). But that's completely different from assuming his answer is being given in bad faith.
There are other answers here that say some variation of the same thing -- that real mastery takes quite a long time. I don't see any reason why this answer should be singled out for being criticized in this way -- other than the fact that jacquesm is very high on the leaderboard and this position makes him a target of gratuitous nonsense of this sort.
If it takes 'decades', you might as well give up now, if you're considering a job opportunity that uses a different language than the one you're used to.
"A lifetime" is not a helpful response in what an honest question like this, because usually everyone realizes that full mastery on the level of compiler implementer is not what is implied by such a question .
Did you miss the part "to a level where you can start contributing to a project with it?" In OPs question?
Is that really decades?