Yes! I'm a big fan, and have even tested the theory out on myself to great effect. I learned French using the series "French in Action", along with certain other sources of input. I was watching and rewatching each episode, and listening on repeat to its audio which I downloaded. Less than a year in I was listening to the radio, and now, two years in, I can listen and read to almost anything online. I'm behind on more casual speech and slang, but am planning to move to France so that I may finally get it.
Meanwhile, I have hardly ever had the chance to speak or write it. I seriously doubt that I've "output" more than a hundred sentences. Perhaps I've spoken some gibberish sentences now and then on my my way to work, but never for any practical purpose. Yet my comprehension is doing okay!
To answer your question: I don't know, but probably at least 4 hours of listening a day in the beginning, gradually decreasing.
The first few months were intense, I remember. My prioritizing was detrimental to a couple of other aspects of life. Every day I wanted to get in some "active study", i.e. watching the current few episodes of FIA over again, trying to understand more this time of the latest one I was working on. And any free moment I had, walking somewhere or going shopping, I'd pop my earbuds in and listen in a more laid back way.
In the very beginning I also figured that my ear and brain needed practice at simply differentiating all these weird new vowels and sounds, so I played French noises almost constantly, including at night and when working. In hindsight, I think only the night time sounds were a smart decision.
Yet despite my dedication being a lot and the project affecting my life, I was always somehow ashamed of my lack of conviction and effort, when hearing about other learners doing "AJAT" or equivalents (All Japanese, All the Time). There are crazier stories our there!
The sad thing also is that I don't think it would be necessary to go so hard at ot to accomplish what I have. An hour or two a day of more optimal input (what I claim is so hard to find), might get you a really long way in the same amount of time. I do not know. My experiment, however, was testing whether I could acquire a language without speaking it, which it did confirm at least to me.
Meanwhile, I have hardly ever had the chance to speak or write it. I seriously doubt that I've "output" more than a hundred sentences. Perhaps I've spoken some gibberish sentences now and then on my my way to work, but never for any practical purpose. Yet my comprehension is doing okay!