I have never seen an adult being exposed just one hour per day to a foreign language over three month, and then suddenly producing grammatically correct meaning

I have never seen an adult being exposed just one hour per day to a foreign language over three month, and then suddenly producing grammatically correct meaningful sentences. And note that we are talking here about passive exposition. No teacher, no corrections, no feedback, no interactions, no carefully crafted pedagogical input, not even slowed down audio. If you know such an adult, please tell them to write a blog about it. I am curious. I have met many adults in multilanguage environments and even some who, as a kind of scientific experiment, specifically tried that approach, without much success though (after some weeks, they could recognize some words and fixed expressions). I don't say that an adult cannot learn a foreign language by immersion. Actually, there are probably hundreds of millions of such people in the world (take any uneducated worker who is forced to work in a foreign country to help his family survive at home). But the conditions are very different (they get feedback and interactions) and the quality of the result, too. The speed and quality with which kids just absorb a new language like sponges is unmatched.