How long does it take someone to go from B1 german to B2, and from B2 to C1?
i can’t give you a straight up answer to this question because it’s entirely dependent on the person in question and their situation.
there are 3 main factors that affect how long it takes someone to progress in a language:
1. how similar the language is to their native language and any other languages they already speak or have experience with
2. how much time they put into studying and using the language and how often
3. how much they push themself outside of their comfort zone (i.e. being exposed to content that is above their current level and that they don’t yet fully understand)
personally as a native english speaker it took me about 3 years to get to c1, studying for at least half an hour a day for the first 2 years and exposing myself to material designed for native speakers (music, movies, books, radio, social media, etc.) and interacting with them (skype, text messaging, face to face, email) for several hours every day, but for another native english speaker who takes german classes at school and only does the bare minimum when it comes to using the language (doing their homework and participating in a few hours of class a week) it might take them 3 years just to reach an a2 level.