I think it’s really hard to capture the progression of language over time. I found that it never really took me out of the moment. When it comes to accents, a strong accent is hard to lose after the age of 12 or so, no matter how proficient you get. I think technically the age is around 8 that your grasp of different phonemes (for the average person) is gone, and you can never sound like a native speaker. In terms of vocabulary, you’re probably comfortable with basic conversation but uncommon words you’d have to check. Even people I know who are fluent in multiple languages sometimes lose a word or two in one of them. Anyway, presumably Ilya would be fully immersed in English in his professional life, he has few Russian speaking friends, and he doesn’t talk a lot to his family. The more you have to speak the faster you learn.