This means not just C2 level, and it doesn't just mean you understand almost everything you hear (though of course that is part of it), but that you're at the point where listening is completely effortless, where you have to expend virtually no energy to understand something, even with extremely colloquial speech, slurred/heavily reduced speech, low audio quality etc. Where you are able to easily follow group conversations with many people constantly interrupting each other.
At the point, psychologically, where listening in the language just feels the same as listening to your native language.
Of course, maybe the question itself is just impossible to answer. I very much doubt there have been any scientific studies to investigate this (though I would love to be proven wrong!), as there aren't many practical purposes for most people; C2 level listening is probably more than enough for the vast majority of jobs or university programs (and probably even less than C2, in many cases), and the question as I have phrased it might not even be scientifically testable anyway. And even immigrants who eventually desire native-level listening comprehension will probably be satisfied in most cases by "merely" reaching an advanced level and letting it gradually improve from that over the years.
Even here, most people probably have not reached this level unless they immigrated to another country. With perhaps the one major exception being some who have learned English as a second language, but considering the way most people these days learn English (with classes in school and constant exposure to English over many years), those people probably don't have much to say about this.
But since this is a language learning forum, there very well may be some here who have done this and were crazy enough to obsessively log their time spent, or at least have some thoughts about it. It would also be helpful even if your estimates are not "from zero" but e.g. from B1 or B2 or something. Like I said, personal anecdotes are fine, and I don't expect the numbers to be very precise (even something like "3000 hours", as long as there is something behind it, would be very interesting to hear).
And yes, I know it depends on your native language, what languages you already speak blah blah blah.
For the record, I will just say I don't think the answer to this is "however long it takes to reach C2", or even (god forbid) "C2 is better than a native". I'm not trying to open that can of worms, but I think you could easily pass C2 while not being even remotely close to what I am talking about. This is true both in terms of the actual exams (where that is pretty obviously the case) and for the more vague self-assessment grid, where C2 listening is described as:
I have no difficulty in understanding any kind of spoken language, whether live or broadcast, even when delivered at fast native speed, provided I have some time to get familiar with the accent.