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.
