I was able to get C2 in Reading, C1 in Listening, B2 in Writing and B2 in Speaking after 10 months, took me 2 attempts at the exam though, 1st one I had a B1 in speaking. I only studied with teachers for about 50 days (after I failed the 1st exam). I was also working full time, but studying about 4 hours of French per day. I think that the teachers made a difference because I hired several tutors from different countries which helped me to understand and be able to speak to French speakers from Africa and Europe.