Define “reasonable”…
Something like 20 years ago, I learned PHP in 2 days… meaning, I could write better PHP than anyone else on the team.
(not to diss on the team, one was a Java guy who left shortly afterwards, the others were a couple interns, while I had the power of something like 10 years of coding experience… and a PHP cheatsheet-booklet)
That sounds less like you learned the language to a high standard, and more that you were already a good programmer in general terms and everyone else on your team barely knew what they were doing.
Ultimately if you can write good code in one language, you can probably also do it in another (especially with access to cheat sheets), but I still wouldn’t call using a cheat sheet having “learned” a language.
Of course it’s all relative and subjective - which is the whole point , one person may consider just being able to write syntactically correct statements as having “learned” a language. Where others might expect a deep knowledge of the language features, standard libraries, and best design practices (this is the side that I personally lean, which I maintain can’t be done in 2 days)
