in loco = on location, on site
Naturally, it depends on what level you have when you arrive in the country.
If you are attending classes, or learning online when you are in the country, you will progress much, much faster than if you try to just "pick it up from my environment". That doesn't work....especially not for men, it seems. Regular lessons/practice is the key both outside and inside the country!
If you arrive with an A2 understanding of English, let's say, expect to reach B1 rather quickly....but B2 after about 3-5 years. C1 you will reach after about 7-8 years in the country (with semi-regular lessons or practice)
I am basing this on my own experiences learning a language in country, and the experiences of the thousands of other learners I see around me every day.