Oh, I knew what you meant with the dreams, and it's those dreams I'm talking about. It's not uncommon for students in intensive language programs to have them, regardless of level. It has more to do with how hard they're studying than how well they speak. I think part of the reason for the conflict in our viewpoints is that there are so many variables involved. For an adult learner to go from zero to B2 in six months from immersion alone would be remarkable. With the addition of regular instruction it's possible. For learners who have already had instruction, six months of immersion could get them to C1, if they were dedicated. Also, teachers tend to be cynical about immersion experiences because we know that especially for exchange students they are rarely perfect immersion--students tend to gravitate towards others
who speak their language, even when studying in a
different country.
NB: The Common European Framework for languages has six levels, starting with A1, which is complete beginner, to C2, which is high-level proficiency. Most foreign language learners don't progress past B2, which is the level at which people can communicate pretty effectively, and which is often the minimum requirement for studying at a foreign university. (At least, in the UK and France that is the minimum). I can tell you from personal experience that the jump fro
B2 to C1 is extremely difficult, much more so than A2 to B1, or even C1 to C2. It's known as "the intermediate plateau." Immersion would certainly make the transition easier, but to be really effective it would need to be supplemented by formal instruction.