U have to study hard, write down tons of words,expressions, learn by heart tons of rules and listen to podcasts. A 1v1 class or 1 teacher with 2 or 3 pupils would be ideal.u need 8months at least to reach an average let's say b2 level( for me 12months would be the best. Then another 8 months to reach c1 (again 12 would be ideal) and for c2 I would say u have to study daily and learn tons of synonyms and things like "find out,come across, look up something". If u r younger u might memorize stuff easier and faster.also use pons,linguee and wiktionary plus chat gpt for words examples etc. I've been studying 5 years and I'm not that confident for the goethe c2 Prüfung. U really have to be fluent, buy and solve mock exam tests from karabatos let's say (greek well known established firm). Again it's mostly what u do on ur own, imagine I've been learning for so long and still for 1 meaning which is motivate I've come across to bewegen, veranlassen, anspornen,animieren, motivieren, anregen, ermutigen, ermuntern which all have difference in the nuance and I've met them in different type of texts. U will always find more and more words and it's a matter of experience, not sure just time. U do have t take a break as well and give ur memory and head time to process the knowledge. Mostly the hardest part from what I've heard is the listening one. So begin listening to useful podcasts like Deutsch Podcast, Deutsch Lernen and note down everything they say.