I'd guess about 2 weeks but I'd be interested to hear other people's take on this. I'd break that down as:
1. The syntax: an hour
2. The rules of the language & the grammar: Under 2 days
3. Ability to write all the idiomatic constructs from muscle memory: Under a week. Timebox yourself to 1.5 days reading the compiler, std library, popular big projects in the language and dedicate the rest of the time to writing
4. The landscape: the common tools, popular and essential libraries, some tidbits of latest news in the community: Under 3 days
You're clearly NOT going to be an expert in the language after 2 weeks but you're going to be productive as an already experienced developer.