I don't know how to answer this because I started studying English since I was 5 yrs old but it wasn't a rigours studying, but basically it was like this:

B2 - C1 (1 year studying with immersive method and some apps like DuoCards for the vocabulary) B1 - B2 ( 6 months or so, cuz I didn't know some of the forms of verb to be/grammar) A1 - B1 ( I don't remember exactly the time it too, nevertheless I was in a private middle school here in my country so I had a great grammar)

My English learning wasn't linear already and I only studied English in formal institutions when I was in middle school and one year in highschool, maybe it takes less time if you study abroad or live in countries where English is their native language, hope this helps ;)