[bryanrasmussen]

people often move to places with a language that is needed to exist in that place and then manage to learn that language. I would say becoming proficient in a language with small effort and no especial facility in languages should take no more than 2 years. With a good deal of intensive effort, focus and natural 3-6 months.

this also depends a lot on the language, many of the Romance languages are relatively easy to learn, many of the Nordic ones seem quite difficult.

Difficulty of course may depend on what you are moving from to what you will be using.

[neffy]

Abilities to do this range widely with individuals.

Leaving that aside, there is a concept of linguistic distance from people´s first language, which makes it easier or harder to learn another language. French and English are very close to each other, as are Arabic and Hebrew, the Scandinavian languages are also all very close to each other (a little further from English), except for Finnish of course, which is kind of out there on its own. There is a nice recent paper looking at this in the academic context:

https://www.sciencedirect.com/science/article/pii/S004873332...

Essentially how far your native tongue is from the language you are attempting to learn will drastically affect how long it takes to learn it for most people.

[082349872349872]

For the other direction, based on a couple of centuries of data: https://www.state.gov/foreign-language-training/

Some languages take english L1 speakers 3x as long to learn as others.

I agree with GP's 2 years for Cat I-II languages (assuming you're mostly living there and learning the language in your spare time, not intensively).