Learn the language by living there!
I'm thinking of spending some time in Turkey to improve my Turkish after finishing my Turkish tree. Have you ever tried this way to learn a language? How long does it take to learn a language in this way? By learning, I mean reaching B2 or C1.
My French being limited to spoken until my family moved to Québec at the start of fourth grade, I went from basically spelling by sound (and extremely entry-level reading skills) to a point where my French reading and writing was at least on par with my English skills within about three months.
I would never have seen that coming; I thought it would take me years to catch up. If you're interested in writing in French and you can get your hands on even just a few elementary-school French workbooks, they turned out to be very effective for me.
I assume you mean "B2 or C1"?
I suspect the only estimates I would rely on would come from people familiar with the learning of Turkish specifically (you might do better asking this in the Turkish forum). I was around French people who learned Spanish solely through immersion. Maybe it took them 2 or 3 months to be reasonably comfortable (which might not have included quite all the grammar for getting a B2, however), but unless you happen to be a native speaker of another Turkic language, I have no reason whatsoever to suppose that time scale is relevant.
Would you be taking formal language classes or just trying to pick it up through exposure? Relying solely on the second method, I think the the best guess of the time period would be "never."