If you've been developing for about 10 years (and hopefully should have used (at the very least) 3 languages by then), you would have realised by now it does not matter on the language/library you use - you will achieve average proficiency sooner than 2-3 months.
There are also those of us, however, who have spent the past 10 years coding the same crap, and learning a new technology would be like starting from scratch.
So, I speculate that you can work out the ability of a developer in an unknown language by multiplying the number of known languages by the total experience time. Roughly, as there are many other factors.
If you've been developing for about 10 years (and hopefully should have used (at the very least) 3 languages by then), you would have realised by now it does not matter on the language/library you use - you will achieve average proficiency sooner than 2-3 months.
There are also those of us, however, who have spent the past 10 years coding the same crap, and learning a new technology would be like starting from scratch.
So, I speculate that you can work out the ability of a developer in an unknown language by multiplying the number of known languages by the total experience time. Roughly, as there are many other factors.
Recruiters (and their clients) do not know this.