Five days in Prague, really four. Yet I greatly improved both in my ability to comprehend Czech and my ability to speak Czech. This was not just because of these few days in Prague. It was the result of one year of work. For at least the first 8 months I only listened and read. Then I started speaking with our online tutors at LingQ, a few times a week. With a couple of months to go I stepped this up to five hours a week of on on one discussions. In Prague I was surrounded by the language, and managed to engage in Czech discussions 6-7 hours a day for four straight days. The final evening I had dinner with a Czech couple in their home. We had a pleasant discussion on topics rainging from travel, to politics, to ecology and more. It was painless and seamless. Victory.
I will adopt the same technique for my next language Korean, and then more beyond that.