B1 to B2 is a big step. Bigger than A1-2 to B1. B1 is basically just understanding people who speak slowly and clearly and being able to participate in everyday life situations. So a very gradual progression. B2 is already being able to do professional work in that language. This is what will take considerable time and active study. But it is also not a requirement for naturalisation in most countries. And many natural born citizens also never reach C1-2 level of proficiency either. So if you're B2, you may as well be counted as fully integrated.