How long does it take to advance one level in the CEFR system?

Many factors can influence how fast one person learns a foreign language. The type of instruction, the learning environment, and many others. Usually, it takes a few years of dedication and commitment to reach a C1+ level in a foreign language. Of course, there are exceptions and some learners advance more quickly. 

Here is a curve that provides a rough idea of how long it takes to advance one level in the CEFR system: