I'm pretty sure that really depends on the person involved. It's a bit like learning a new (spoken) language. There are a few people who can pick up a new language in a few weeks, others spend a lifetime.

It also depends on the problem you are working on. If you've done something similar before or using one of your standard constructs, you will get fluent quite fast. If you do something totally different, you might have to use subsets of the programming language you seldomly, if ever, have used before. Same goes for using your mother tongue: You might have used it for decades fluently, but that doesn't automatically mean you can write a medical record or a set of laws in it without effort, as these often use special forms of the language with words you mostly never use in your day-to-day communication.